add gdb line test for thread start and current thread markers

these tests do not presently pass because they regressed in the
qregularexpression port 8d8062fe

as for exmaple observed in https://bugs.kde.org/show_bug.cgi?id=423222
which lacks the marker lines

at a glance exactMatch->hasMatch wasn't a good port and so the line
falls into the CRAP regex' `0x[0-9a-f]+.*|` which would match line
containing a hex number