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

tests/test-narrow.t

@@ -51,7 +51,7 b" Error if '.' or '..' are in the director"
   [255]
 
 Names with '.' in them are OK.
-  $ hg clone --narrow ssh://user@dummy/master $RANDOM --include a/.b/c
+  $ hg clone --narrow ssh://user@dummy/master should-work --include a/.b/c
   requesting all changes
   adding changesets
   adding manifests