Adding YMD case

Adding a YMD case which appears to be failing.

Author: JAMSUPREME

test/test-suite/groups/function-tomillis/case012.json

@@ -0,0 +1,6 @@
+{
+    "expr": "$toMillis(\"20180205\", \"[Y0001][M01][D01]\")",
+    "dataset": "dataset5",
+    "bindings": {},
+    "result": 1520208000000
+}