[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
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 76e4af8..d33453c 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -140,6 +140,9 @@
     },
     {
       "name": "FrameworksNetTests"
+    },
+    {
+      "name": "NetHttpCoverageTests"
     }
   ],
   "mainline-presubmit": [