tests: AT_SAVE_SPECIAL_FILES / AT_RESTORE_SPECIAL_FILES.

tests: AT_SAVE_SPECIAL_FILES / AT_RESTORE_SPECIAL_FILES.

Some of our macros play with expout and other Autotest special files,
which may break their callers (e.g., currently TESTSUITEFLAGS='248
BISON_TEST_XML=1' fails).

There is already some support for this.  Expand it to be ready to use
it elsewhere.

* tests/local.at (AT_RESTORE_SPECIAL_FILES, AT_SAVE_SPECIAL_FILES)
(at_save_special_files, at_restore_special_files): New.
(AT_BISON_CHECK_NO_XML): Use them.