Fix testsuite failures with bash 5.2.

Fix testsuite failures with bash 5.2.

Bash v5.2 includes several new optimizations to the number of
subshells used for various constructs; as a side effect, the value of
SHLVL is less stable than it used to be.  Add SHLVL to the list of
built-in shell variables with unstable values, to be ignored when
checking for inappropriate differences to the shell environment
before and after a macro invocation / between two configure runs.

Problem and solution reported by Xi Ruoyao in
https://lists.gnu.org/archive/html/autoconf/2022-09/msg00015.html
Problem also reported by Bruce Dubbs in
https://lists.gnu.org/archive/html/bug-autoconf/2022-09/msg00010.html

* tests/local.at (_AT_CHECK_ENV, AT_CONFIG_CMP): Ignore changes in
  value of SHLVL.
Copyright-paperwork-exempt: yes