Rename ash_mus_unittests builder step to "ash_unittests --mus"

testing/buildbot/chromium.chromiumos.json

 {
   "Linux ChromiumOS Builder": {
     "additional_compile_targets": [
       "All"
     ]
   },
   "Linux ChromiumOS Ozone Tests (1)": {
     "gtest_tests": [
       {
         "swarming": {
@@ skipping 27 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "ash_content_unittests"
       },
       {
         "args": [
           "--mus",
-          "--test-launcher-filter-file=../../testing/buildbot/filters/ash_mus_unittests.filter"
+          "--test-launcher-filter-file=../../testing/buildbot/filters/ash_unittests_mus.filter"
         ],
-        "name": "ash_mus_unittests",
+        "name": "ash_unittests --mus",
         "override_isolate_target": "ash_unittests",
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "ash_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
@@ skipping 1140 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "wm_unittests"
       }
     ]
   }
 }