selftests: drv-net: psp: fix test names in ipver_test_builder()

selftests: drv-net: psp: fix test names in ipver_test_builder()

test_case will only take on the formatted name after being
called. This does not work with the way ksft_run() currently
works. Assign the name after the test_case is created.

Fixes: 81236c74dba6 ("selftests: drv-net: psp: add test for auto-adjusting TCP MSS")
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20251216-psp-test-fix-v1-2-3b5a6dde186f@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>