Add: Acceptance test for rendering quotes in mustache

Ref: https://dev.cfengine.com/issues/6516
(cherry picked from commit 51ca2ac5964d8a2966f728c96e15d654ab13c514)
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.

(cherry picked from commit b2e5b9b1abf6291cb5da5fef16c1047476b8f7d1)