Allow passing arguments to bootstrap_test.py

Previous, it used the built-in test runner, which doesn't support options unless they're manually passed in the script.

src/bootstrap/bootstrap_test.py

@@ -1,4 +1,6 @@
-"""Bootstrap tests"""
+"""Bootstrap tests
+
+Run these with `x test bootstrap`, or `python -m unittest bootstrap_test.py`."""
 
 from __future__ import absolute_import, division, print_function
 import os
@@ -120,15 +122,3 @@ def test_set_codegen_backends(self):
         build = self.serialize_and_parse(["--enable-full-tools"])
         self.assertNotEqual(build.config_toml.find("codegen-backends = ['llvm']"), -1)
 
-if __name__ == '__main__':
-    SUITE = unittest.TestSuite()
-    TEST_LOADER = unittest.TestLoader()
-    SUITE.addTest(doctest.DocTestSuite(bootstrap))
-    SUITE.addTests([
-        TEST_LOADER.loadTestsFromTestCase(VerifyTestCase),
-        TEST_LOADER.loadTestsFromTestCase(GenerateAndParseConfig),
-        TEST_LOADER.loadTestsFromTestCase(ProgramOutOfDate)])
-
-    RUNNER = unittest.TextTestRunner(stream=sys.stdout, verbosity=2)
-    result = RUNNER.run(SUITE)
-    sys.exit(0 if result.wasSuccessful() else 1)

src/bootstrap/test.rs

@@ -2664,7 +2664,12 @@ impl Step for Bootstrap {
     /// Tests the build system itself.
     fn run(self, builder: &Builder<'_>) {
         let mut check_bootstrap = Command::new(&builder.python());
-        check_bootstrap.arg("bootstrap_test.py").current_dir(builder.src.join("src/bootstrap/"));
+        check_bootstrap
+            .arg("-m")
+            .arg("unittest")
+            .arg("bootstrap_test.py")
+            .current_dir(builder.src.join("src/bootstrap/"))
+            .args(builder.config.test_args());
         try_run(builder, &mut check_bootstrap).unwrap();
 
         let host = builder.config.build;