Don't pass --test-args to python -m unitest

The args for unittest and cargo test are mutually incompatible. Suggest that people use `python -m unittest ...` manually instead.

This also changes `bootstrap_test.py` to be easier to run standalone; see the commit for details.

src/bootstrap/bootstrap_test.py

@@ -1,6 +1,6 @@
 """Bootstrap tests
 
-Run these with `x test bootstrap`, or `python -m unittest bootstrap_test.py`."""
+Run these with `x test bootstrap`, or `python -m unittest src/bootstrap/bootstrap_test.py`."""
 
 from __future__ import absolute_import, division, print_function
 import os
@@ -12,6 +12,10 @@ import sys
 
 from shutil import rmtree
 
+# Allow running this from the top-level directory.
+bootstrap_dir = os.path.dirname(os.path.abspath(__file__))
+# For the import below, have Python search in src/bootstrap first.
+sys.path.insert(0, bootstrap_dir)
 import bootstrap
 import configure
 
@@ -131,8 +135,13 @@ class BuildBootstrap(unittest.TestCase):
 
         parsed = bootstrap.parse_args(args)
         build = serialize_and_parse(configure_args, parsed)
-        build.build_dir = os.environ["BUILD_DIR"]
-        build.build = os.environ["BUILD_PLATFORM"]
+        # Make these optional so that `python -m unittest` works when run manually.
+        build_dir = os.environ.get("BUILD_DIR")
+        if build_dir is not None:
+            build.build_dir = build_dir
+        build_platform = os.environ.get("BUILD_PLATFORM")
+        if build_platform is not None:
+            build.build = build_platform
         return build.build_bootstrap_cmd(env), env
 
     def test_cargoflags(self):

src/bootstrap/test.rs

@@ -2682,8 +2682,9 @@ impl Step for Bootstrap {
             .args(["-m", "unittest", "bootstrap_test.py"])
             .env("BUILD_DIR", &builder.out)
             .env("BUILD_PLATFORM", &builder.build.build.triple)
-            .current_dir(builder.src.join("src/bootstrap/"))
-            .args(builder.config.test_args());
+            .current_dir(builder.src.join("src/bootstrap/"));
+        // NOTE: we intentionally don't pass test_args here because the args for unittest and cargo test are mutually incompatible.
+        // Use `python -m unittest` manually if you want to pass arguments.
         try_run(builder, &mut check_bootstrap).unwrap();
 
         let host = builder.config.build;