Refactor test helpers (#1336)

* Refactor test helpers

* Combine common codes from `parse` and `build`, and dispatch using
`Mode`
* Remove `toNotBuild` and `toNotParse` and use `expect.not`
* Remove `Warning` and instead mock `console.warn`
* Add `expect.toHavePassed` and check whether parse/build succeeded
lazily
* Improve failed test `message`:
  - Use color
  - Print stack traces(excluding internals)
  - Print diff
  - Follow jest matcher output style

* Update helpers.js

* Remove toBeTruthy checks

getParsed throws an error if parsing fails.

* Used tagged literals

* Use .toHaveLength

* Use tagged literals

* Use to{Build,Parse}Like where possible

* Remove compareParseTree

* Use snapshot where possible

* Join into one line where <88 chars

* Revert console.warn() to throw an error

Merge `expectToWarn` to `expectKaTeX`, like
`expect.toFailWithParseError`

* Remove call to console.warn from stack traces

* Fix merge errors

* Remove `getTree`

* Move `_getBuilt` into `getBuilt`
* Move default settings and tagging literal support in to `getParsed` 
and `getBuilt`
* Remove stack traces clean-up
* Remove `toHavePassed` matcher

* Extract `expected` string construction into `printExpectedResult`

test/__snapshots__/katex-spec.js.snap

@@ -585,6 +585,28 @@ exports[`A font parser \\boldsymbol should inherit mbin/mrel from argument 1`] =
 ]
 `;
 
+exports[`A parse tree generator generates a tree 1`] = `
+[
+  {
+    "type": "supsub",
+    "mode": "math",
+    "value": {
+      "type": "supsub",
+      "base": {
+        "type": "mathord",
+        "mode": "math",
+        "value": "\\\\sigma"
+      },
+      "sup": {
+        "type": "textord",
+        "mode": "math",
+        "value": "2"
+      }
+    }
+  }
+]
+`;
+
 exports[`A parser that does not throw on unsupported commands should build katex-error span for other type of KaTeX error 1`] = `
 {
   "attributes": {