testsuite: Handle `balanced ASCII quotes' correctly in m4 errors.

testsuite: Handle `balanced ASCII quotes' correctly in m4 errors.

AT_CHECK_M4 was _documented_ to convert

   m4: cannot open `foo': No such file or directory

to

   m4: cannot open 'foo': No such file or directory

but the regexp that was supposed to do this didn’t work.

Fix the regexp, and remove a workaround for the bug in one test
(yes, we had code in our own testsuite to work around bugs in our
own testsuite!)

* tests/local.at (AT_CHECK_M4): Fix regex for “cannot open” messages.
* tests/tools.at (autom4te cache): Remove workaround for buggy regex
  in AT_CHECK_M4.