unittests/base: Allow init method to fail

This adds a new keyword argument to the init method, `allow_fail`. When
set to True (default is False) then a failure to configure is not an
error, and output is still returned. This can be useful for cases where
we expect initialization to fail, and want to check the output.

unittests/baseplatformtests.py

@@ -112,13 +112,18 @@ class BasePlatformTests(TestCase):
         newdir = os.path.realpath(newdir)
         self.change_builddir(newdir)
 
-    def _print_meson_log(self):
+    def _get_meson_log(self) -> T.Optional[str]:
         log = os.path.join(self.logdir, 'meson-log.txt')
         if not os.path.isfile(log):
-            print(f"{log!r} doesn't exist")
+            print(f"{log!r} doesn't exist", file=sys.stderr)
-            return
+            return None
         with open(log, encoding='utf-8') as f:
-            print(f.read())
+            return f.read()
+
+    def _print_meson_log(self) -> None:
+        log = self._get_meson_log()
+        if log:
+            print(log)
 
     def tearDown(self):
         for path in self.builddirs:
@@ -160,7 +165,15 @@ class BasePlatformTests(TestCase):
              default_args=True,
              inprocess=False,
              override_envvars=None,
-             workdir=None):
+             workdir=None,
+             allow_fail: bool = False) -> str:
+        """Call `meson setup`
+
+        :param allow_fail: If set to true initialization is allowed to fail.
+            When it does the log will be returned instead of stdout.
+        :return: the value of stdout on success, or the meson log on failure
+            when :param allow_fail: is true
+        """
         self.assertPathExists(srcdir)
         if extra_args is None:
             extra_args = []
@@ -180,10 +193,12 @@ class BasePlatformTests(TestCase):
             try:
                 returncode, out, err = run_configure_inprocess(self.meson_args + args + extra_args, override_envvars)
             except Exception as e:
-                # Don't double print
+                if not allow_fail:
-                if str(e) != 'Configure failed':
                     self._print_meson_log()
-                raise
+                    raise
+                out = self._get_meson_log()  # Best we can do here
+                err = ''  # type checkers can't figure out that on this path returncode will always be 0
+                returncode = 0
             finally:
                 # Close log file to satisfy Windows file locking
                 mesonbuild.mlog.shutdown()
@@ -198,15 +213,18 @@ class BasePlatformTests(TestCase):
                 print(out)
                 print('Stderr:\n')
                 print(err)
-                raise RuntimeError('Configure failed')
+                if not allow_fail:
+                    raise RuntimeError('Configure failed')
         else:
             try:
                 out = self._run(self.setup_command + args + extra_args, override_envvars=override_envvars, workdir=workdir)
             except SkipTest:
                 raise SkipTest('Project requested skipping: ' + srcdir)
             except Exception:
-                self._print_meson_log()
+                if not allow_fail:
-                raise
+                    self._print_meson_log()
+                    raise
+                out = self._get_meson_log()  # best we can do here
         return out
 
     def build(self, target=None, *, extra_args=None, override_envvars=None):