Add better default error message for mismatched strings

[maier49]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code matches the style guide
• Unit or Functional tests are included in the PR

Description:
This adds a default message for when assertRender fails comparing strings. In contrast to the error in #56, this is the error for the same failure with this change.

AssertionError: Render unexpected: Expected "foo" to equal "bar"
  at <_build\src\support\assertRender.js:141:17>
  at Array.forEach  <native>
  at assertChildren  <_build\src\support\assertRender.js:138:20>
  at Object.assertRender [as default]  <_build\src\support\assertRender.js:202:21>
  at Test.string children unequal [as test]  <tests\unit\support\assertRender.ts:126:5>
  at <node_modules\intern\lib\Test.js:191:24>
  at <node_modules\intern\browser_modules\dojo\Promise.ts:393:15>
  at runCallbacks  <node_modules\intern\browser_modules\dojo\Promise.ts:11:11>
  at <node_modules\intern\browser_modules\dojo\Promise.ts:317:4>
  at run  <node_modules\intern\browser_modules\dojo\Promise.ts:237:7>

Resolves #56

[codecov]

Codecov Report

Merging #57 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #57   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files          10       10           
  Lines         692      692           
  Branches      161      162    +1     
=======================================
  Hits          680      680           
  Misses          4        4           
  Partials        8        8

Impacted Files	Coverage Δ
src/support/assertRender.ts	97.5% <100%> (ø)	⬆️

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