[Cronet] NetHttpCoverageTests presubmit SLO fixes

- Adds NetHttpCoverageTests to postsubmit. This would allow us
to run tests with @SkipPresubmit in post submit.
- Don't jarjar com.android.testutil so that the annotation can
still be found by the testrunner.

Bug: b/293843426
Test: th
Change-Id: I9e792d338515acba798092bbc4c615aef2cac47a

Cronet/tests/mts/jarjar_excludes.txt

+# Exclude some test prefixes, as they can't be found after being jarjared.
+com\.android\.testutils\..+
 # jarjar-gen can't handle some kotlin object expression, exclude packages that include them
 androidx\..+
 kotlin\.test\..+

TEST_MAPPING

@@ -140,6 +140,9 @@
     },
     {
       "name": "FrameworksNetTests"
+    },
+    {
+      "name": "NetHttpCoverageTests"
     }
   ],
   "mainline-presubmit": [