[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
2 files changed