mtest: fix rebuilding correct target list before running tests

When manually specifying a target on the CLI, which does not have any
test deps, we  calculate that there are no deps to rebuild, we run ninja
without any targets, and this invokes the default "all" rule and maybe
builds a few thousand targets that this specific test does not need.

In such a case we simply should not run ninja at all, since we don't
want to rebuild anything.

mesonbuild/mtest.py

@@ -1972,6 +1972,11 @@ def convert_path_to_target(path: str) -> str:
             depends.update(d)
             targets.update(intro_targets[d])
 
+    if tests and not targets:
+        # We want to build minimal deps, but if the subset of targets have no
+        # deps then ninja falls back to 'all'.
+        return True
+
     ret = subprocess.run(ninja + ['-C', wd] + sorted(targets)).returncode
     if ret != 0:
         print(f'Could not rebuild {wd}')

unittests/platformagnostictests.py

@@ -92,4 +92,8 @@ def test_mtest_rebuild_deps(self):
         self._run(self.mtest_command)
         self.clean()
 
+        with self.assertRaises(subprocess.CalledProcessError):
+            self._run(self.mtest_command + ['runner-without-dep'])
+        self.clean()
+
         self._run(self.mtest_command + ['runner-with-exedep'])