Add a soak-eunit target to Makefile
Add a soak-eunit target to Makefile

This allows developers to run eunit in a loop until a test crashes. This
can be helpful for fixing flaky tests that are failing due to timing
related issues. For instance:

    make soak-eunit apps=ddoc_cache suites=ddoc_cache_lru_test

This command would run that specific test module repeatedly until it hit
a timing condition that caused a failure. And yes, I used this change to
fix that failure. :D

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