gdb/testsuite: Avoid infinite loop in gdb.reverse/step-reverse.exp

gdb/testsuite: Avoid infinite loop in gdb.reverse/step-reverse.exp

This testcase sometimes gets stuck in a loop for hours when running in our
CI.  The problem is that due to an issue unrelated to reverse debugging the
inferior exits early, and because of the overly generic ".*" pattern the
testcase keeps sending the "next" command without noticing that the
inferior is gone.

gdb_test_multiple has a pattern to detect that "The program is not being
run.", but since it is placed after the patterns from the caller it won't
be triggered.  It also has a timeout pattern but because it is triggered
between successful matches, each time the test matches the '-re -wrap ".*"'
this counts as a successful match and the timeout is reset.

Since the test binary is compiled with debug information, fix by changing
one of the generic patterns to match entering the main function and the
other one to match the source code line number that is shown by GDB right
after the "step" command.

Also, as a precaution add a maximum number of times the "next" command will
be sent.

Co-Authored-By: Tom de Vries <tdevries@suse.de>
Reviewed-By: Bruno Larsen <blarsen@redhat.com>
Approved-By: Tom de Vries <tdevries@suse.de>