test-lib: fix non-portable pattern bracket expressions

test-lib: fix non-portable pattern bracket expressions

Use a '!' character to start a non-matching pattern bracket
expression, as specified by POSIX in Shell Command Language section
2.13.1 Patterns Matching a Single Character [1].

I used '^' instead in three places in the previous three commits, to
verify that the arguments of the '--stress=' and '--stress-limit='
options and the values of various '*_PORT' environment variables are
valid numbers.  With certain shells, at least with dash (upstream and
in Ubuntu 14.04) and mksh, this led to various undesired behaviors:

  # error message in case of a valid number
  $ ~/src/dash/src/dash ./t3903-stash.sh --stress=8
  error: --stress=<N> requires the number of jobs to run

  # not the expected error message
  $ ~/src/dash/src/dash ./t3903-stash.sh --stress=foo
  ./t3903-stash.sh: 238: test: Illegal number: foo

  # no error message at all?!
  $ mksh ./t3903-stash.sh --stress=foo
  $ echo $?
  0

Some other shells, e.g. Bash (even in posix mode), ksh, dash in Ubuntu
16.04 or later, are apparently happy to accept '^' just as well.

[1] http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_13

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>