unit-tests: Make sure plugins in the builddir are loaded

When running the tests in GDB the working directory apparently is
different.  With the relative path used previously the plugins would not
be found and those installed on the system would get used.

src/libstrongswan/tests/Makefile.am

@@ -52,7 +52,7 @@ tests_SOURCES = tests.h tests.c \
 tests_CFLAGS = \
   -I$(top_srcdir)/src/libstrongswan \
   -I$(top_srcdir)/src/libstrongswan/tests \
-  -DPLUGINDIR=\""$(top_builddir)/src/libstrongswan/plugins\"" \
+  -DPLUGINDIR=\""$(abs_top_builddir)/src/libstrongswan/plugins\"" \
   -DPLUGINS=\""${s_plugins}\"" \
   @COVERAGE_CFLAGS@
 

src/libtls/tests/Makefile.am

@@ -11,7 +11,7 @@ tls_tests_CFLAGS = \
 	-I$(top_srcdir)/src/libtls \
 	-I$(top_srcdir)/src/libstrongswan \
 	-I$(top_srcdir)/src/libstrongswan/tests \
-	-DPLUGINDIR=\""$(top_builddir)/src/libstrongswan/plugins\"" \
+	-DPLUGINDIR=\""$(abs_top_builddir)/src/libstrongswan/plugins\"" \
 	-DPLUGINS=\""${s_plugins}\"" \
 	@COVERAGE_CFLAGS@