test/runtests: add MPITEST_IGNORE_OUTPUT

[hzhou]

Pull Request Description

Set environment variable MPITEST_IGNORE_OUTPUT if we want to enable verbose mode such as FI_LOG_LEVEL and prevent unexpected output triggering test failures. The tests will rely on "No Errors" output and exit status for pass/failure.

[skip warnings]

Author Checklist

• [x] Provide Description Particularly focus on why, not what. Reference background, issues, test failures, xfail entries, etc.
• [x] Commits Follow Good Practice Commits are self-contained and do not do two things at once. Commit message is of the form: module: short description Commit message explains what's in the commit.
• [ ] Passes All Tests Whitespace checker. Warnings test. Additional tests via comments.
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[hzhou]

test:mpich/custom env: VERBOSE=1 env: MPITEST_IGNORE_OUTPUT=1

[hzhou]

test:mpich/custom netmod: ch4:ofi env: FI_LOG_LEVEL=info env: MPITEST_IGNORE_OUTPUT=1

[hzhou]

test:mpich/custom netmod: ch4:ofi env: VERBOSE=1 env: FI_LOG_LEVEL=info env: MPITEST_IGNORE_OUTPUT=1

[hzhou]

test:mpich/custom netmod: ch4:ofi env: V=1 env: FI_LOG_LEVEL=info env: MPITEST_IGNORE_OUTPUT=1