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

test/mathml-spec.js

@@ -0,0 +1,52 @@
+/* global expect: false */
+/* global it: false */
+/* global describe: false */
+
+import buildMathML from "../src/buildMathML";
+import parseTree from "../src/parseTree";
+import Options from "../src/Options";
+import Settings from "../src/Settings";
+import Style from "../src/Style";
+
+const defaultSettings = new Settings({});
+
+const getMathML = function(expr, settings) {
+    const usedSettings = settings ? settings : defaultSettings;
+
+    let startStyle = Style.TEXT;
+    if (usedSettings.displayMode) {
+        startStyle = Style.DISPLAY;
+    }
+
+    // Setup the default options
+    const options = new Options({
+        style: startStyle,
+    });
+
+    const built = buildMathML(parseTree(expr, usedSettings), expr, options);
+
+    // Strip off the surrounding <span>
+    return built.children[0].toMarkup();
+};
+
+describe("A MathML builder", function() {
+    it('should generate the right types of nodes', () => {
+        expect(getMathML("\\sin{x}+1\\;\\text{a}")).toMatchSnapshot();
+    });
+
+    it('should make prime operators into <mo> nodes', () => {
+        expect(getMathML("f'")).toMatchSnapshot();
+    });
+
+    it('should generate <mphantom> nodes for \\phantom', () => {
+        expect(getMathML("\\phantom{x}")).toMatchSnapshot();
+    });
+
+    it('should use <munderover> for large operators', () => {
+        expect(getMathML("\\displaystyle\\sum_a^b")).toMatchSnapshot();
+    });
+
+    it('should use <msupsub> for regular operators', () => {
+        expect(getMathML("\\textstyle\\sum_a^b")).toMatchSnapshot();
+    });
+});