RFR: 8372248: gtest istream.coverage depends on istream.basic

[Axel Boldt-Christmas]

These two GTests have a strong dependency that `istream.coverage` is ran after `istream.basic`. This goes against the intended design of GTests. (They should be independent).

As such I propose we merge this into one test.

I kept the two `VERBOSE` variables separate. However I am not sure I understand their purpose.
Currently changing `VERBOSE_TEST` to `true` will cause the test to fail, (not all cases are covered). And the value have of `VERBOSE_COVERAGE` has no effect. What is observed:
 * `(VERBOSE_TEST: false, VERBOSE_COVERAGE: false) -> Success`
 * `(VERBOSE_TEST: false, VERBOSE_COVERAGE:  true) -> Success`
 * `(VERBOSE_TEST:  true, VERBOSE_COVERAGE: false) -> Failure`
 * `(VERBOSE_TEST:  true, VERBOSE_COVERAGE:  true) -> Failure`

But I kept the original behaviour, just merged into one test case rather than two.

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Depends on: https://git.openjdk.org/jdk/pull/28409

Commit messages:
 - gtest istream.coverage depends on istream.basic

Changes: https://git.openjdk.org/jdk/pull/28418/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28418&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372248
  Stats: 8 lines in 2 files changed: 0 ins; 4 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/28418.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28418/head:pull/28418

PR: https://git.openjdk.org/jdk/pull/28418