tests: add test for delete-if-present and append-if-missing

when there is existing `key`

tests/vmcheck/test-kernel-args.sh

@@ -178,6 +178,17 @@ vm_rpmostree kargs --delete-if-present=PACKAGE3=TEST3 --unchanged-exit-77 || rc=
 assert_streq $rc 77
 echo "ok exit 77 when unchanged kargs with unchanged-exit-77"
 
+# Test append-if-missing and delete-if-present for existing key
+vm_rpmostree kargs --append-if-missing=PACKAGE3=TEST3
+vm_rpmostree kargs > kargs.txt
+assert_file_has_content_literal kargs.txt 'PACKAGE3=TEST3'
+rpm-ostree kargs --append-if-missing=PACKAGE3=NEWTEST
+vm_rpmostree kargs > kargs.txt
+assert_file_has_content_literal kargs.txt 'PACKAGE3=NEWTEST'
+rpm-ostree kargs --delete-if-present=PACKAGE3=TEST || rc=$?
+assert_streq $rc 0
+vm_rpmostree kargs --delete-if-present=PACKAGE3=TEST3
+
 # Test for 'rpm-ostree kargs --editor'.
 vm_rpmostree kargs > kargs.txt
 assert_not_file_has_content_literal kargs.txt 'nonexisting'