tests: Use lv_abs_top_builddir instead of bare abs_top_builddir

As stated in the comment above introduction of the lv_abs_top_builddir
variable, older automake doesn't provide abs_top_builddir variable.
Hence, we are creating our own one with lv_ prefix. However, when
exporting env variables to the tests, the variables are not evaluated
but only substituted. Hence:

  LIBVIRT_DRIVER_DIR="$(abs_top_builddir)/src/.libs"

is set to "/src/.libs" with old automake (even though we *think* we've
set the $abs_top_builddir variable just a few line above).
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

tests/Makefile.am

@@ -345,7 +345,7 @@ TESTS_ENVIRONMENT =				\
   CONFIG_HEADER="`cd '$(top_builddir)'; pwd`/config.h"	\
   PATH="$(path_add)$(PATH_SEPARATOR)$$PATH"	\
   SHELL="$(SHELL)"				\
-  LIBVIRT_DRIVER_DIR="$(abs_top_builddir)/src/.libs" \
+  LIBVIRT_DRIVER_DIR="$(lv_abs_top_builddir)/src/.libs" \
   LIBVIRT_AUTOSTART=0				\
   LC_ALL=C					\
   VIR_TEST_EXPENSIVE=$(VIR_TEST_EXPENSIVE)	\