[gdb/testsuite] Emit unresolved for unknown proc

Since commit 26783bce "[gdb/testsuite] Don't abort testrun for invalid
command in test-case" we don't abort the testrun when encountering an invalid
command.  However, since we don't report errors in the summary, there's a
chance that the error goes unnoticed.

Make the invalid command error more visible by marking the test-case
unresolved, such that we have f.i.:
...
PASS: gdb.python/py-breakpoint.exp: test_bkpt_internal: Test watchpoint write
UNRESOLVED: gdb.python/py-breakpoint.exp: test_bkpt_eval_funcs: \
  testcase aborted due to invalid command name: gdb_py_test_multiple
ERROR: tcl error sourcing py-breakpoint.exp.
ERROR: invalid command name "gdb_py_test_multiple"
    while executing
  ...
                === gdb Summary ===

nr of expected passes            56
nr of unresolved testcases       1
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-06-29  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (unknown): Make test-case unresolved.