Tweak test_hashtab_thrash test to allow the critical threads to execute.

Depending on certain conditions it was possible for the hashtab counting thread
to starve other threads, preventing them from executing in the expected fashion.
This change adds a sleep to allow the others to do what they need to do. While
this doesn't thrash the hashtab as much as previously, it at least works.

(closes issue ASTERISK-22276)
Reported by: Matt Jordan
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Merged revisions 396619 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 396620 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@396621 f38db490-d61c-443f-a65b-d21fe96a405b
Tweak comment for why usleep is used.
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Merged revisions 396656 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 396657 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@396658 f38db490-d61c-443f-a65b-d21fe96a405b