Add new coverage_skia.isolate

Coverage is different from the other recipes because it needs both skia_repo and test artifacts. The primary reason is: the path to the out directory is compiled into the binary, which is why the compile and test steps need to be combined.

The isolate file will be used from the updated coverage recipe in https://codereview.chromium.org/1862713002/

BUG=skia:5159
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1893503003

infra/bots/coverage_skia.isolate

@@ -0,0 +1,6 @@
+{
+  'includes': [                                                                 
+    'skia_repo.isolate',                                                        
+    'test_skia.isolate',
+  ],
+}