add --coverage-continuous-mode=1 for test_env.py and enable for mac-code-coverage

Enabling a clang coverage mode that profile counter updates are continuously synced to a file. This will help reduce the coverage profile dumping overhead and reduce the corrupted issue when process killed or crashed.

[1] https://clang.llvm.org/docs/SourceBasedCodeCoverage.html#running-the-instrumented-program

Bug: 1223230
Change-Id: Ib97e7e876d3380f8d4dc3179a99f81b75539971c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3179667
Reviewed-by: Dirk Pranke <[email protected]>
Commit-Queue: Kuan Huang <[email protected]>
Cr-Commit-Position: refs/heads/main@{#933108}
3 files changed