test: correctly clean up test artifacts

Remove the artifact files indicating test result (testok, failed, and
skipped) just before running the test so we always get the latest and
most relevant result instead of incorrectly consuming previous results.

Discovered in https://github.com/systemd/systemd/pull/15378#issuecomment-616801873

test/test-functions

@@ -234,6 +234,7 @@ run_qemu() {
 
     CONSOLE=ttyS0
 
+    rm -f "$initdir"/{testok,failed,skipped}
     # make sure the initdir is not mounted to avoid concurrent access
     cleanup_initdir
     umount_loopback
@@ -362,6 +363,7 @@ $QEMU_OPTIONS \
 # success), or 1 if nspawn is not available.
 run_nspawn() {
     [[ -d /run/systemd/system ]] || return 1
+    rm -f "$initdir"/{testok,failed,skipped}
 
     local _nspawn_cmd=(
         --register=no
@@ -2014,7 +2016,6 @@ test_setup() {
 
 test_run() {
     mount_initdir
-    rm -f "$initdir"/{testok,failed,skipped}
 
     if [ -z "$TEST_NO_QEMU" ]; then
         if run_qemu "$1"; then