mtest: fix logic bug that broke tests where the cmd is a Windows found program

In this case, the test fname might have an implicit extension and cannot
be found by `os.path.isfile()`.

We cannot use `shutil.which()` to handle platform differences, because
not all test fnames are executable -- for example Java jars.

The test representation does have an "is built" attribute which in
theory should work here, because all built targets definitely have their
full filename known to Meson, but it turns out to be misnamed. Rename it
correctly and add an actual "is built" attribute to check.

Tests which aren't built by Meson can be assumed to exist without
consulting their existence on the filesystem.

Fixes #10027

mesonbuild/backend/backends.py

@@ -224,6 +224,7 @@ class TestSerialisation:
     protocol: TestProtocol
     priority: int
     cmd_is_built: bool
+    cmd_is_exe: bool
     depends: T.List[str]
     version: str
     verbose: bool
@@ -1147,6 +1148,7 @@ class Backend:
                                    t.is_parallel, cmd_args, t.env,
                                    t.should_fail, t.timeout, t.workdir,
                                    extra_paths, t.protocol, t.priority,
+                                   isinstance(exe, build.Target),
                                    isinstance(exe, build.Executable),
                                    [x.get_id() for x in depends],
                                    self.environment.coredata.version,

mesonbuild/mtest.py

@@ -1349,18 +1349,18 @@ class SingleTestRunner:
 
     def _get_test_cmd(self) -> T.Optional[T.List[str]]:
         testentry = self.test.fname[0]
-        if self.options.no_rebuild and not os.path.isfile(testentry):
+        if self.options.no_rebuild and self.test.cmd_is_built and not os.path.isfile(testentry):
             raise TestException(f'The test program {testentry!r} does not exist. Cannot run tests before building them.')
         if testentry.endswith('.jar'):
             return ['java', '-jar'] + self.test.fname
         elif not self.test.is_cross_built and run_with_mono(testentry):
             return ['mono'] + self.test.fname
-        elif self.test.cmd_is_built and self.test.is_cross_built and self.test.needs_exe_wrapper:
+        elif self.test.cmd_is_exe and self.test.is_cross_built and self.test.needs_exe_wrapper:
             if self.test.exe_wrapper is None:
                 # Can not run test on cross compiled executable
                 # because there is no execute wrapper.
                 return None
-            elif self.test.cmd_is_built:
+            elif self.test.cmd_is_exe:
                 # If the command is not built (ie, its a python script),
                 # then we don't check for the exe-wrapper
                 if not self.test.exe_wrapper.found():