mariadb: add missing test dep on libevent

This bit me on multibuild [1] . It's included  in
tests/CMakeLists.txt. Also make building of the tests conditional.

[1] [ 72%] Building C object tests/CMakeFiles/mysql_client_test.dir/mysql_client_test.c.o
cd /var/tmp/paludis/build/dev-db-mariadb-5.5.33a-r1/work/C/32/tests && /usr/bin/x86_64-pc-linux-gnu-gcc  -DHAVE_CONFIG_H -DMYSQL_CLIENT -Wall -m32 -march=native -pipe -O2 -floop-block -floop-interchange -floop-strip-mine -O3 -DNDEBUG -DDBUG_OFF -DMY_PTHREAD_FASTMUTEX=1 -I/var/tmp/paludis/build/dev-db-mariadb-5.5.33a-r1/work/C/32/include -I/var/tmp/paludis/build/dev-db-mariadb-5.5.33a-r1/work/mariadb-5.5.33a/include    -m32 -o CMakeFiles/mysql_client_test.dir/mysql_client_test.c.o   -c /var/tmp/paludis/build/dev-db-mariadb-5.5.33a-r1/work/mariadb-5.5.33a/tests/mysql_client_test.c
/usr/lib64/gcc/x86_64-pc-linux-gnu/4.8.2/../../../../x86_64-pc-linux-gnu/bin/ld: warning: skipping incompatible /usr/lib64/gcc/x86_64-pc-linux-gnu/4.8.2/../../../libevent.so while searching for event
/usr/lib64/gcc/x86_64-pc-linux-gnu/4.8.2/../../../../x86_64-pc-linux-gnu/bin/ld: warning: skipping incompatible /usr/lib/libevent.so while searching for event
/usr/lib64/gcc/x86_64-pc-linux-gnu/4.8.2/../../../../x86_64-pc-linux-gnu/bin/ld: error: cannot find -levent

Change-Id: If25ed332b3e39dee267dfe80b3b8591690af0a36