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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}