Actually check TEST_SHARD_STATUS_FILE has been touched

Adds the missing check that a test runner running a sharded test
actually supports sharding. Previously, if it didn't, each shard would
silently run all tests.

RELNOTES: If sharding is requested for a test but the test runner does
not advertise support for test sharding by touching the
`TEST_SHARD_STATUS_FILE`, the tests will fail instead of silently
running all tests in each shard.

src/test/py/bazel/bazel_windows_test.py

@@ -449,6 +449,31 @@ def testZipUndeclaredTestOutputs(self):
     self.assertTrue(os.path.exists(output_file))
     self.assertFalse(os.path.exists(output_zip))
 
+  def testTestShardStatusFile(self):
+    self.CreateWorkspaceWithDefaultRepos('WORKSPACE')
+    self.ScratchFile(
+        'BUILD',
+        [
+          'sh_test(',
+          '  name = "foo_test",',
+          '  srcs = ["foo.sh"],',
+          '  shard_count = 2,',
+          ')',
+        ],
+    )
+    self.ScratchFile('foo.sh')
+
+    exit_code, _, stderr = self.RunBazel(['test', '//:foo_test'])
+    # Check for "tests failed" exit code
+    self.AssertExitCode(exit_code, 3, stderr)
+    self.assertIn("Sharding requested, but the test runner did not advertise "
+                  "support for it by touching TEST_SHARD_STATUS_FILE.", stderr)
+
+    self.ScratchFile('foo.sh', ['touch "$TEST_SHARD_STATUS_FILE"'])
+
+    exit_code, _, stderr = self.RunBazel(['test', '//:foo_test'])
+    self.AssertExitCode(exit_code, 0, stderr)
+
 
 if __name__ == '__main__':
   unittest.main()

src/test/shell/bazel/bazel_test_test.sh

@@ -997,4 +997,24 @@ EOF
   expect_log "<testcase name=\"x\""
 }
 
+function test_shard_status_file_checked() {
+  cat <<'EOF' > BUILD
+sh_test(
+    name = 'x',
+    srcs = ['x.sh'],
+    shard_count = 2,
+)
+EOF
+  touch x.sh
+  chmod +x x.sh
+
+  bazel test //:x --test_output=errors &> $TEST_log \
+      && fail "expected failure"
+  expect_log "Sharding requested, but the test runner did not advertise support for it by touching TEST_SHARD_STATUS_FILE."
+
+  echo 'touch "$TEST_SHARD_STATUS_FILE"' > x.sh
+  bazel test //:x --test_output=errors &> $TEST_log \
+      || fail "expected success"
+}
+
 run_suite "bazel test tests"