tests: fix spurious failure due to Solaris XPG4 sh bug

tests: fix spurious failure due to Solaris XPG4 sh bug

On Solaris 10, the /usr/xpg4/bin/sh shell seems unable to execute
a shell script named 'script':

  $ touch script
  $ /bin/sh script; echo status: $? # As expected.
  status: 0
  $ /usr/xpg4/bin/sh script; echo status: $? # Weirdness follows.
  /usr/xpg4/bin/sh: script: cannot execute
  status: 1

This was causing a spurious testsuite failure for users which have
/usr/xpg4/bin in $PATH before /bin and /usr/bin.  Fix that.

* tests/m4sh.at (Configure re-execs self with CONFIG_SHELL): Rename
the m4sh-produced script to 'script2', to avoid the just-described
issue.