Remove unused tests/configure.ac

Remove unused tests/configure.ac

tests/configure.ac was introduced 15 years ago by commit
d7f8d0caa7a357f9f4765e5dc93255f5057eba2e.  However, the ability to build
tests as a separate project was broken by the same author 4 years later
by commit 22359e265395fd2f8267190ef97f5417770e6206, if not earlier.

An attempt to run autoreconf in tests would currently fail
with the following automake error:
automake: error: cannot open < config/eu.am: No such file or directory

Apparently, nobody builds tests as a separate project for more than 10
years, so cleanup the remains of that unused and broken code.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>