tests: remove use of bashism $RANDOM to fix test-narrow.t on non-bash shells
This test passed on our workstations, including on Macs, so we never
noticed, but it fails on both the Linux and FreeBSD buildbots. Today I
learned about a bash feature, wherein $RANDOM gives a random int every
time you read it.
check-code rule to follow.
Differential Revision: https://phab.mercurial-scm.org/D2236
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