tests: update `x' quotes to 'x' to match gnulib outputs.

In the (extremely!) long time since this branch was last active,
GNU is moving towards upright quotes exclusively, including
several of the files we use from gnulib -- which breaks many of
the test cases.
* doc/m4.texi: Fix quotes in many example tests extracted to
tests/generated.m4.
* tests/builtins.at, tests/freeze.at, tests/macros.at,
tests/modules.at, tests/null.at, tests/options.at,
tests/others.at: Fix quotes in many expected output comparisons.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>