mtest: move the detection of rebuild compatibility out into the entry point

What we are mainly doing here is checking that the options make sense,
or fixing that up if they don't. And in the next commit we will want to
do that by also checking the build object.

mesonbuild/mtest.py

@@ -1582,12 +1582,6 @@ class TestHarness:
         if self.options.no_rebuild:
             return
 
-        if not (Path(self.options.wd) / 'build.ninja').is_file():
-            print('Only ninja backend is supported to rebuild tests before running them.')
-            # Disable, no point in trying to build anything later
-            self.options.no_rebuild = True
-            return
-
         self.ninja = environment.detect_ninja()
         if not self.ninja:
             print("Can't find ninja, can't rebuild test.")
@@ -2104,6 +2098,12 @@ def run(options: argparse.Namespace) -> int:
     b = build.load(options.wd)
     setup_vsenv(b.need_vsenv)
 
+    if not options.no_rebuild:
+        if not (Path(options.wd) / 'build.ninja').is_file():
+            print('Only ninja backend is supported to rebuild tests before running them.')
+            # Disable, no point in trying to build anything later
+            options.no_rebuild = True
+
     with TestHarness(options) as th:
         try:
             if options.list: