Rename ash_mus_unittests builder step to "ash_unittests --mus"

testing/buildbot/chromium.fyi.json

 {
   "Android Asan Builder Tests (dbg)": {
     "gtest_tests": [
       {
         "args": [
           "--tool=asan"
         ],
         "swarming": {
           "can_use_on_swarming_builders": false
         },
@@ skipping 11635 matching lines @@
     ]
   },
   "Mojo ChromiumOS": {
     "additional_compile_targets": [
       "mash:all"
     ],
     "gtest_tests": [
       {
         "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"
       },
       {
         "args": [
           "--override-use-software-gl-for-tests",
           "--test-launcher-filter-file=../../testing/buildbot/filters/mojo.fyi.browser_tests.filter"
@@ skipping 1444 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": false
         },
         "test": "views_unittests"
       }
     ]
   }
 }