Make `font-subset` a standalone `run_tests.py` variant. (#162016)

Closes https://github.com/flutter/flutter/issues/161538.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/dc42bcf2ec708ed3899d33fa02a077220b71201e

DEPS

diff --git a/DEPS b/DEPS
index 832a67d..01879ea 100644
--- a/DEPS
+++ b/DEPS
@@ -239,7 +239,7 @@
 deps = {
   'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'f77e9ba17d8600a091772001ba29fa964bc80d6a',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'e33e4f0476bd613502fb1724b3996f09b25fb01e',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'dc42bcf2ec708ed3899d33fa02a077220b71201e',
 
   'flutter/engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',

commits.json

diff --git a/commits.json b/commits.json
index bd609c6..0dd6dc5 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
-  "flutter":"e33e4f0476bd613502fb1724b3996f09b25fb01e",
+  "flutter":"dc42bcf2ec708ed3899d33fa02a077220b71201e",
   "flutter/engine/src/flutter/third_party/dart":"f77e9ba17d8600a091772001ba29fa964bc80d6a"
 }
\ No newline at end of file