Skip coverage tracer import test on FreeBSD

There is no pre-built coverage wheel for FreeBSD on PyPI, so
python-rules' built-in test runners have to fall back on the pure-Python
tracing implementation on FreeBSD. Don't run the `tracer` import test in
`//test:python3_coverage_test` on FreeBSD - we expect it to fail.

Fixes #274.

Author: chrisnovakovic

test/python_coverage_test.py

@@ -21,6 +21,7 @@ def test_can_import_coverage(self):
         import coverage
         self.assertIsNotNone(coverage)
 
+    @unittest.skipIf(sys.platform.startswith("freebsd"), "built-in test runners do not contain a tracer module compatible with this platform")
     def test_can_import_tracer(self):
         """Test we can import the binary tracer module."""
         from coverage import tracer