tests: Disable the migrations tests

* tests/Makefile.am (SUBDIRS): Remove migrations.
* configure.ac (AC_CONFIG_FILES): Remove migrations Makefile.

The tests introduced with commit defbc70b require some non-portable
tools like mktemp and basename. They further fail with "make
distcheck". Removed for now.

• Signed-off-by: Werner Koch <wk@gnupg.org>