[gdb/testsuite] Fix gdb.dap/type_check.exp with older python

[gdb/testsuite] Fix gdb.dap/type_check.exp with older python

On openSUSE Leap 15.4 with system python 3.6, I run into:
...
(gdb) python check_everything()^M
(gdb) FAIL: gdb.dap/type_check.exp: type checker
...

In check_everything, the hasattr test fails silently:
...
def check_everything():
    # Older versions of Python can't really implement this.
    if hasattr(typing, "get_origin"):
...
and that makes the gdb_test in the test-case fail.

Fix this by emitting UNSUPPORTED instead in check_everything, and detecting
this in the test-case.

Tested on x86_64-linux.