test: Run some XTS5 integration tests against Xvfb if possible.

By default the tests will be skipped.  However, if you set XTEST_DIR
to the repo of a built X Test Suite and PIGLIT_DIR to a piglit repo
(no build necessary), make check will run piglit's xts-render tests
against Xvfb.

We could run more of XTS5, but I haven't spent the time identifying
what additional subset would be worth running, since much of it is
only really testing the client libraries.  We want to make sure that
we keep the runtime down, and this subset of the test suite took 92
seconds according to piglit.

Reviewed-by: Adam Jackson <ajax@redhat.com>

test/Makefile.am

@@ -12,8 +12,23 @@ endif
 endif
 check_LTLIBRARIES = libxservertest.la
 
-TESTS=$(noinst_PROGRAMS)
-TESTS_ENVIRONMENT = $(XORG_MALLOC_DEBUG_ENV)
+if XVFB
+XVFB_TESTS = scripts/xvfb-piglit.sh
+endif
+
+SCRIPT_TESTS = \
+	$(XVFB_TESTS) \
+	$(NULL)
+
+TESTS = \
+	$(noinst_PROGRAMS) \
+	$(SCRIPT_TESTS) \
+	$(NULL)
+
+TESTS_ENVIRONMENT = \
+	XSERVER_DIR=$(abs_top_builddir) \
+	$(XORG_MALLOC_DEBUG_ENV) \
+	$(NULL)
 
 AM_CFLAGS = $(DIX_CFLAGS) @XORG_CFLAGS@
 AM_CPPFLAGS = $(XORG_INCS)
@@ -139,5 +154,9 @@ endif
 libxservertest_la_DEPENDENCIES = $(libxservertest_la_LIBADD)
 endif
 
-EXTRA_DIST = ddxstubs.c
+EXTRA_DIST = \
+	$(SCRIPT_TESTS) \
+	scripts/xinit-piglit-session.sh \
+	ddxstubs.c \
+	$(NULL)