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

• Provide Description
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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