Fix MathML output for ' and large operators with limits (#788)

Summary: (@kevinbarabash)
This diff extracts MathML builder tests out into a separate file and
updates them to use jest snapshot testing.  It updates the output of
prime ' from using identifier nodes <mi> to operator nodes <mo>.  It
also updates large operators with limits to use munderover instead
of msupsub. I added an option to remove unnecessary mrows to buildGroup.
Right now it's only used for by groupTypes.supsub. I'll see if it can be used
elsewhere (everywhere?) in a follow up PR.

Test Plan:
- make test w/o errors
- verify mathml snapshots contain the desired markup

package.json

@@ -28,6 +28,7 @@
     "express": "^4.14.0",
     "glob": "^7.1.1",
     "jest": "^20.0.4",
+    "jest-serializer-html": "^4.0.0",
     "js-yaml": "^3.3.1",
     "jspngopt": "^0.2.0",
     "less": "~2.7.1",
@@ -48,6 +49,9 @@
     "match-at": "^0.1.0"
   },
   "jest": {
+    "snapshotSerializers": [
+      "jest-serializer-html"
+    ],
     "testMatch": [
       "**/test/*-spec.js"
     ],