tests: limit autotest-functionname generation to sane characters

tests: limit autotest-functionname generation to sane characters

Some (older) versions of bash seem to be allergic to a method named
"aptautotest_grep_^apt" (note the caret). Unlikely that we are going to
write autotests for such commands so we could just skip those, but lets
instead just use "normal" characters in the names and strip the rest as
we already did with the (arguable more common) '-'.