Run unit tests with both layout 2013 and layout 2020 (#30032)

Since #29950, unit tests were only running with the legacy layout, and
there was no way to run them for layout 2020.

This patch makes './mach test-unit' run unit tests for both.

Also doing some changes so that the layout 2020 floats.rs tests compile.

python/servo/command_base.py

@@ -927,7 +927,6 @@ class CommandBase(object):
         libsimpleservo=False,
         debug_mozjs=False, with_debug_assertions=False,
         with_frame_pointer=False, without_wgl=False,
-        with_layout_2020=False, with_layout_2013=False,
         **_kwargs
     ):
         env = env or self.build_env()

python/servo/testing_commands.py

@@ -167,7 +167,7 @@ class MachCommands(CommandBase):
     @CommandArgument('--nocapture', default=False, action="store_true",
                      help="Run tests with nocapture ( show test stdout )")
     @CommandBase.build_like_command_arguments
-    def test_unit(self, test_name=None, package=None, bench=False, nocapture=False, with_layout_2020=False, **kwargs):
+    def test_unit(self, test_name=None, package=None, bench=False, nocapture=False, **kwargs):
         if test_name is None:
             test_name = []
 
@@ -200,6 +200,8 @@ class MachCommands(CommandBase):
         self_contained_tests = [
             "background_hang_monitor",
             "gfx",
+            "layout_2013",
+            "layout_2020",
             "msg",
             "net",
             "net_traits",
@@ -208,10 +210,6 @@ class MachCommands(CommandBase):
             "servo_config",
             "servo_remutex",
         ]
-        if with_layout_2020:
-            self_contained_tests.append("layout_2020")
-        else:
-            self_contained_tests.append("layout_2013")
         if not packages:
             packages = set(os.listdir(path.join(self.context.topdir, "tests", "unit"))) - set(['.DS_Store'])
             packages |= set(self_contained_tests)
@@ -255,7 +253,6 @@ class MachCommands(CommandBase):
             "bench" if bench else "test",
             args,
             env=env,
-            with_layout_2020=with_layout_2020,
             **kwargs)
 
     @Command('test-content',