unittests: allow running with pytest/unittest directly

Currently the unittests are not runnable with pytest or unittest
without going through the run_unittests.py wrapper.

This has that downside that the common "pytest ..." fails and integration
with things like VSCode fails too.

To work around that we set everything that is needed to run the tests
in __init__.py and run_unittests is only one more variant to invoke them
by providing different defaults and settings.

To make sure that pytest/unittest discover and run_unittests don't diverge
implement an automatic test discovery in run_unittests to avoid hardcoding
the tests to run there. There shouldn't be any functional changes.

run_unittests.py

@@ -12,42 +12,9 @@ import subprocess
 import os
 import unittest
 
-import mesonbuild.mlog
-import mesonbuild.depfile
-import mesonbuild.dependencies.base
-import mesonbuild.dependencies.factory
-import mesonbuild.compilers
-import mesonbuild.envconfig
-import mesonbuild.environment
 import mesonbuild.coredata
-import mesonbuild.modules.gnome
 from mesonbuild.mesonlib import python_command, setup_vsenv
-import mesonbuild.modules.pkgconfig
 
-from unittests.allplatformstests import AllPlatformTests
-from unittests.cargotests import CargoVersionTest, CargoCfgTest, CargoLockTest
-from unittests.darwintests import DarwinTests
-from unittests.failuretests import FailureTests
-from unittests.linuxcrosstests import LinuxCrossArmTests, LinuxCrossMingwTests
-from unittests.machinefiletests import NativeFileTests, CrossFileTests
-from unittests.rewritetests import RewriterTests
-from unittests.taptests import TAPParserTests
-from unittests.datatests import DataTests
-from unittests.internaltests import InternalTests
-from unittests.linuxliketests import LinuxlikeTests
-from unittests.pythontests import PythonTests
-from unittests.subprojectscommandtests import SubprojectsCommandTests
-from unittests.windowstests import WindowsTests
-from unittests.platformagnostictests import PlatformAgnosticTests
-
-def unset_envs():
-    # For unit tests we must fully control all command lines
-    # so that there are no unexpected changes coming from the
-    # environment, for example when doing a package build.
-    varnames = ['CPPFLAGS', 'LDFLAGS'] + list(mesonbuild.compilers.compilers.CFLAGS_MAPPING.values())
-    for v in varnames:
-        if v in os.environ:
-            del os.environ[v]
 
 def convert_args(argv):
     # If we got passed a list of tests, pass it on
@@ -100,15 +67,34 @@ def setup_backend():
     os.environ['MESON_UNIT_TEST_BACKEND'] = be
     sys.argv = filtered
 
+def import_test_cases(suite: unittest.TestSuite) -> list:
+    '''
+    Imports all test classes into the current module and returns their names
+    '''
+    classes = set()
+    for test in suite:
+        if isinstance(test, unittest.TestSuite):
+            classes.update(import_test_cases(test))
+        elif isinstance(test, unittest.TestCase):
+            import importlib
+            mod = importlib.import_module(test.__module__)
+            class_name = test.__class__.__name__
+            test_class = getattr(mod, class_name)
+            classes.add(class_name)
+            setattr(sys.modules[__name__], class_name, test_class)
+    return sorted(classes)
+
+def discover_test_cases() -> list:
+    current_dir = os.path.dirname(os.path.realpath(__file__))
+    loader = unittest.TestLoader()
+    suite = loader.discover(os.path.join(current_dir, 'unittests'), '*tests.py', current_dir)
+    if loader.errors:
+        raise SystemExit(loader.errors)
+    return import_test_cases(suite)
+
 def main():
-    unset_envs()
+    cases = discover_test_cases()
     setup_backend()
-    cases = ['InternalTests', 'DataTests', 'AllPlatformTests', 'FailureTests',
-             'PythonTests', 'NativeFileTests', 'RewriterTests', 'CrossFileTests',
-             'TAPParserTests', 'SubprojectsCommandTests', 'PlatformAgnosticTests',
-
-             'LinuxlikeTests', 'LinuxCrossArmTests', 'LinuxCrossMingwTests',
-             'WindowsTests', 'DarwinTests']
 
     try:
         import pytest # noqa: F401

unittests/__init__.py

@@ -0,0 +1,23 @@
+import os
+
+import mesonbuild.compilers
+from mesonbuild.mesonlib import setup_vsenv
+
+def unset_envs():
+    # For unit tests we must fully control all command lines
+    # so that there are no unexpected changes coming from the
+    # environment, for example when doing a package build.
+    varnames = ['CPPFLAGS', 'LDFLAGS'] + list(mesonbuild.compilers.compilers.CFLAGS_MAPPING.values())
+    for v in varnames:
+        if v in os.environ:
+            del os.environ[v]
+
+def set_envs():
+    os.environ.setdefault('MESON_UNIT_TEST_BACKEND', 'ninja')
+
+def init():
+    setup_vsenv()
+    unset_envs()
+    set_envs()
+
+init()