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