ci: use proper naming format for piglit tests

Update the piglit ignore_tests and sed pattern to use the real format
which is path/to/test, with @ instead of /.

This will allow us to pass the file directly into piglit, just like we
do with deqp ;-)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Elie Tournier <elie.tournier@collabora.com>

ci/previous_results/gl_host_softpipe/piglit_gles2/ignore_tests.txt

@@ -1 +1 @@
-arb_blend_func_extended-fbo-extended-blend-pattern_gles2
+spec@arb_blend_func_extended@arb_blend_func_extended-fbo-extended-blend-pattern_gles2

ci/run_test_suite.sh

@@ -122,7 +122,8 @@ compare_previous()
    # which enables us to ignore tests by driver
    # BUT: We're not able to get the driver name and
    # use it to disambiguate between HW-based drivers
-   IGNORE_TESTS=$(cat $IGNORE_TESTS_FILE 2>/dev/null)
+   # Piglit tests use @ as separator for path/to/test
+   IGNORE_TESTS=$(sed "s/\@/\//g" $IGNORE_TESTS_FILE 2>/dev/null)
 
    # Avoid introducing changes while doing this comparison
    TMP_RESULTS=$(mktemp /tmp/virgl_ci_results.XXXXXX)