Extract `mojo_test` - standalone apptest runner in devtools.

mojo/tools/data/apptests

Index: mojo/tools/data/apptests
diff --git a/mojo/tools/data/apptests b/mojo/tools/data/apptests
index 1087fd11874c682e49e66eef854db15db7b3aaee..18693f1b286ba9cf3348b2308c8dfce5f1d26f8a 100644
--- a/mojo/tools/data/apptests
+++ b/mojo/tools/data/apptests
@@ -73,7 +73,7 @@ tests = [
   },
 ]
 
-if config.target_os == config.OS_LINUX:
+if target_os == 'linux':
   tests += [
     {
       "test": "mojo:example_apptests",
@@ -92,7 +92,7 @@ if config.target_os == config.OS_LINUX:
     }
   ]
 
-if config.target_os == config.OS_ANDROID:
+if target_os == 'android':
   tests += [
     {
       "test": "mojo:example_apptests",
@@ -111,7 +111,7 @@ if config.target_os == config.OS_ANDROID:
     },
   ]
 
-if config.target_os != config.OS_ANDROID:
+if target_os != 'android':
   tests += [
     {
       "test": "mojo:js_apptests",