style: no longer use backquotes

style: no longer use backquotes

* tests/actions.at, tests/atlocal.in, tests/c++.at, tests/calc.at,
* tests/conflicts.at, tests/existing.at, tests/glr-regression.at,
* tests/input.at, tests/java.at, tests/local.at, tests/sets.at,
* tests/synclines.at, doc/bison.texi, lib/libiberty.h, lib/timevar.h:
Use single quotes.