[gdb/testsuite] Fix test in gdb.python/py-finish-breakpoint.exp

[gdb/testsuite] Fix test in gdb.python/py-finish-breakpoint.exp

With test-case gdb.python/py-finish-breakpoint.exp, we run into:
...
(gdb) python print (finishbp_default.hit_count)
Traceback (most recent call last):
  File "<string>", line 1, in <module>
RuntimeError: Breakpoint 3 is invalid.
Error while executing Python code.
(gdb) PASS: gdb.python/py-finish-breakpoint.exp: normal conditions: \
  check finishBP on default frame has been hit
...

The test producing the pass is:
...
    gdb_test "python print (finishbp_default.hit_count)" "1.*" \
      "check finishBP on default frame has been hit"
...
so the pass is produced because the 1 in "line 1" matches "1.*".

Temporary breakpoints are removed when hit, and consequently accessing the
hit_count attribute of a temporary python breakpoint (gdb.Breakpoint class) is
not possible, and as per spec we get a RuntimeError.

So the RuntimeError is correct, and not specific to finish breakpoints.

The test presumably attempts to match:
...
(gdb) python print (finishbp_default.hit_count)
1
...
but most likely this output was never produced by any gdb version.

Fix this by checking whether the finishbp_default breakpoint has hit by
checking that finishbp_default.is_valid() is False.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR testsuite/31391
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31391