Fix for test of mustache's rendering of quotes.

The test expected variable expansion to be unescaped, but it's escaped
by default; it needs an & at the start of the variable name to
suppress escaping.  I've duly expanded the test to test both escaped
and unescaped variable expansion.
Merge pull request #1903 from ediosyncratic/mustache-fix

redmine6524: Fix for test of mustache's rendering of quotes.