tests: Fix systemd test with --without-driver-modules

tests: Fix systemd test with --without-driver-modules

Every test that makes use of virmock.h (only virsystemdtest as of now)
needs to be linked with -export-dynamic to make sure the LD_PRELOADed
mock library can access its wrap_* symbols. Normally,
DRIVER_MODULE_LDFLAGS variable contains -export-dynamic but when
--without-driver-modules configure option is used, DRIVER_MODULE_LDFLAGS
is empty.

This patch turns on -export-dynamic for all tests unconditionally
regardless on --without-driver-modules. This fixes virsystemdtest and
all future users of virmock.h.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>