tests: quote 'like this', not `like this', as per GCS recommendation

This patch converts the automake testsuite and related files, to
the use of new quoting format 'like this' or "like this" rather
than `like this'.

This is done for consistency with the new recommendations in the GNU
Coding Standards, and, well, because I've come to actually prefer
the '...' and "..." quoting formats over the `...' one.

* tests/README: Update quoting format throughout.  Remove some
"excessive" use of quoting, and throw in minor rewording where
appropriate.
* tests/Makefile.am: Likewise, for comments.
* tests/CheckListOfTests.am: Likewise.
* tests/defs-static.in: Likewise.
* tests/defs: Likewise.
* tests/trivial-test-driver: Likewise.
* Many tests/*.tap, tests/*.test and tests/*.sh files: Likewise,
and for the diagnostic and informational messages as well.
* tests/gen-testsuite-part: Likewise, and for the generated tests
as well.