auto merge of #4090 : mttr/servo/mach_infer_test, r=jdm

Fixes #4086

Lets `./mach test` take a file or directory as an argument, and infers `test-content`, `test-wpt`, or `test-ref` from the file's path. 

Usage Example:
```
$ ./mach test tests/wpt/web-platform-tests/dom/interfaces.html
 0:00.27 LOG: MainThread INFO Using 1 client processes
 ...
```
Note that while there is no functionality for `test-ref` to accept individual files, `./mach test tests/ref` will still trigger `test-ref`.

Passing arguments properly through python would have been ugly (though maybe there are some mach internals I could have taken advantage of), so this instead runs a new mach process and lets the individual test suites manage their arguments like normal.

python/servo/testing_commands.py

@@ -42,10 +42,45 @@ class MachCommands(CommandBase):
         if t:
             return subprocess.call([t] + args, env=self.build_env())
 
+    def infer_test_by_dir(self, params):
+        maybe_path = path.normpath(params[0])
+        mach_command = path.join(self.context.topdir, "mach")
+        args = None
+
+        if not path.exists(maybe_path):
+            print("%s is not a valid file or directory" % maybe_path)
+            return 1
+
+        test_dirs = [
+            (path.join("tests", "content"), "test-content"),
+            (path.join("tests", "wpt"), "test-wpt"),
+        ]
+
+        if path.join("tests", "ref") in maybe_path:
+            # test-ref is the outcast here in that it does not accept
+            # individual files as arguments.
+            args = [mach_command, "test-ref"] + params[1:]
+        else:
+            for test_dir, test_name in test_dirs:
+                if test_dir in maybe_path:
+                    args = [mach_command, test_name, maybe_path] + params[1:]
+                    break
+            else:
+                print("%s is not a valid test file or directory" % maybe_path)
+                return 1
+
+        return subprocess.call(args, env=self.build_env())
+
     @Command('test',
              description='Run all Servo tests',
              category='testing')
-    def test(self):
+    @CommandArgument('params', default=None, nargs="...",
+                     help="Optionally select test based on "
+                          "test file directory")
+    def test(self, params):
+        if params:
+            return self.infer_test_by_dir(params)
+
         test_start = time()
         for t in ["tidy", "unit", "ref", "content", "wpt"]:
             Registrar.dispatch("test-%s" % t, context=self.context)