tests: use regular POSIX shell

wait-on-file requires one POSIX extension (sleep with non-integral
argument), but it doesn't require any bash extensions, so just require a
normal POSIX shell. While here, use consistent formatting without
redundant ;

Differential Revision: https://phab.mercurial-scm.org/D8500

tests/testlib/wait-on-file

@@ -1,4 +1,4 b''
-#!/bin/bash
+#!/bin/sh
 #
 # wait up to TIMEOUT seconds until a WAIT_ON_FILE is created.
 #
@@ -22,13 +22,11 b' if [ $# -eq 3 ]; then'
     create="$3"
 fi
 
-if [ -n "$create" ];
-then
+if [ -n "$create" ]; then
     touch "$create"
     create=""
 fi
-while [ "$timer" -gt 0 ] && [ ! -f "$wait_on" ];
-do
+while [ "$timer" -gt 0 ] && [ ! -f "$wait_on" ]; do
     timer=$(( $timer - 1))
     sleep 0.01
 done