Disable 175-alloc-big-bignums for debuggable

Since that seems to result in interpretation of add() as well.

Test: Treehugger
Change-Id: Idbc008240bb85ba08a6d84e1f91252e14134e425
diff --git a/test/knownfailures.json b/test/knownfailures.json
index 5117859..70572cb 100644
--- a/test/knownfailures.json
+++ b/test/knownfailures.json
@@ -1321,7 +1321,7 @@
     },
     {
         "tests": ["175-alloc-big-bignums"],
-        "variant": "interpreter | interp-ac | trace | no-image",
+        "variant": "interpreter | interp-ac | trace | no-image | debuggable",
         "description": "Interpreting BigInteger.add() is too slow (timeouts)"
     },
     {