configure: make --with-test-suite work

configure: make --with-test-suite work

Our option '--with-test-suite' could have never worked since it was
defined as AC_ARG_ENABLE([with-test-suite], ...), thus working only as
'--enable-with-test-suite', but documented in configure.ac as
AC_HELP_STRING([--with-test-suite], ...).
In my opinion, the help string is as it should be, but the option is
wrong.

The option has been broken since the introduction in commit 3a2fc27.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>