RFR: JDK-8314752: Use google test string comparision macros

[Thomas Stuefe]

Small improvement to use google test's string comparison macros instead of raw C-functions.

This makes the code more readable and provides us with automatic error output in case of string mismatches (so we don't have to print the values via "<<").

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Commit messages:
 - start

Changes: https://git.openjdk.org/jdk/pull/15355/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15355&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314752
  Stats: 84 lines in 14 files changed: 8 ins; 15 del; 61 mod
  Patch: https://git.openjdk.org/jdk/pull/15355.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15355/head:pull/15355

PR: https://git.openjdk.org/jdk/pull/15355