ci/vc4: Skip VS dynamic loops tests that cause GPU hangs.

We've been getting spurious failures from the new VC4 CI, which I believe
are due to this set of tests (which have been showing up along with a GPU
hang report in the list of flaky tests in the failing jobs).  This was a
known issue I had in vc4.

Reviewed-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7998>

.gitlab-ci/deqp-vc4-rpi3-skips.txt

@@ -15,3 +15,14 @@ dEQP-GLES2.functional.fragment_ops.scissor.outside_render_line
 # These are very slow
 dEQP-GLES2.functional.uniform_api.random.3
 dEQP-GLES2.functional.uniform_api.random.79
+
+# Conformance issue: VC4 needs dynamic loops in the VS to cause a
+# shader link failure.
+#
+# The issue is that the HW doesn't have an exec mask at dispatch
+# for the VS, so the shouldn't-be-exec channels have undefined
+# contents and may cause infinite loops, leading to GPU hangs.  The
+# process of GPU hang reset causes flakes in whatever other jobs are
+# running simultaneously, so we can't even leave these in the flakes
+# list for tracking.
+dEQP-GLES2.functional.shaders.loops.*dynamic.*vertex