Tests: Add loopMul.json and loopExp.json to skipped, slow tests

Add these to the list of specially disabled tests during `make test`,
as they are exceptionally slow.

Signed-off-by: Jamie Lokier <jamie@shareable.org>

tests/test_allowed_to_fail.nim

@@ -87,6 +87,8 @@ func slowGSTTests(folder: string, name: string): bool =
 
               # Istanbul slow tests
               "CALLBlake2f_MaxRounds.json",
+              "loopMul.json",
+              "loopExp.json",
 
               ]