[gdb/testsuite] Handle unrecognized escape sequences better in tuiterm

[gdb/testsuite] Handle unrecognized escape sequences better in tuiterm

When encountering an unrecognized escape sequence in Term::accept_gdb_output,
a warning is issued:
...
WARNING: timeout in accept_gdb_output
...
and 0 is returned.

Subsequent calls run into the same problem, so matching doesn't progress.

Consequently, figuring out what the unrecognized escape sequence actually is
depends on analyzing gdb's output as echoed into gdb.log.

In the test added in this commit, gdb (well, a script gdb.tcl emulating gdb)
outputs escape sequence "ESC ( B", which doesn't show up in recognizable form
in gdb.log:
...
foo^M^M
...
and as mentioned the tuiterm screen only show:
...
foo
...
because matching doesn't progress beyond the unrecognized sequence.

Fix this by rewriting accept_gdb_output to match gdb output using a phased
approach that echoes unmatched escape sequences, giving us instead on the
tuiterm screen:
...
foo^[(B
...

[ Since "ESC ( B" is now supported, the test-case has been updated to use
"ESC ( % 5" instead. ]

Tested on x86_64-linux.

PR testsuite/33218
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33218