tests: question mark is extended regex for non-GNU grep.

Accepting \? for at-most-once in basic regex is a GNU grep
extension, not accepted by AIX grep for example.
* tests/libtool.at: Use \{0,1\} instead of ? with GREP.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
tests: do not test undef symbols across shlibs on AIX.

On AIX, undefined symbols across shared libraries can work only
when the main program explicitly exports those symbols. As this
is bad practice anyway and -no-undefined should be preferred, we
skip this.
* tests/template.at: Skip test with undef syms across libraries
on AIX.

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