[gdb/testsuite] Don't abort testrun for invalid command in test-case

Say we add a call to foobar at the end of a test-case, and run the
test-suite.  We'll run into a dejagnu error:
...
ERROR: (DejaGnu) proc "foobar" does not exist.
...
and the test-suite run is aborted.

It's reasonable that the test-case is aborted, but it's not reasonable that
the testsuite run is aborted.

Problems in one test-case should not leak into other test-cases, and they
generally don't.  The exception is the "invalid command name" problem due to
an override of ::unknown in dejagnu's framework.exp.

Fix this by reverting dejagnu's ::unknown override for the duration of each
test-case, using the gdb_init/gdb_finish hooks.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-06-12  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26110
	* lib/gdb.exp (gdb_init): Revert dejagnu's override of ::unknown.
	(gdb_finish): Reinstall dejagnu's override of ::unknown.