[gdb/testsuite] Be more verbose about abort in gdb_breakpoint

[gdb/testsuite] Be more verbose about abort in gdb_breakpoint

I noticed that an abort when setting a breakpoint does not result in more
than:
...
(gdb) break 27^M
FAIL: gdb.a/b.exp: setting breakpoint at 27 (eof)
...

Handle this more verbosely, as is done in gdb_test_multiple, such that we have
instead:
...
(gdb) break 27^M
ERROR: GDB process no longer exists
GDB process exited with wait status 29309 exp9 0 0 CHILDKILLED SIGABRT SIGABRT
UNRESOLVED: gdb.a/b.exp: setting breakpoint at 27 (eof)
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-10-16  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_breakpoint): Handle eof as in gdb_test_multiple.