PHPUnit tests: cater for plugins which disable wptexturize()

[bobbingwide]

I recently updated the bigram plugin to disable wptexturize logic
This function, amongst other things, converts single and double quotes to formatted entities.
Some of the PHPUnit test cases currently expect strings to contain formatted entities.
If the bigram plugin were activated then these would not be formatted by wptexturize, so the string comparison logic would fail.
One solution is to deactivate bigram before running the PHPUnit tests, another is to change the test functions to cater for the possible differences.

I tried writing code to re-enable wptexturize for the PHP Unit tests, but this is not possible with the current version of wptexturize.
There's a TRAC issue which I updated. TRAC #54721