portable: Test pinning a single fstype on an GPT image with verity

This tests a GPT image with a single fstype using verity. This was
broken and fixed by the previous commit.

test/units/TEST-50-DISSECT.dissect.sh

@@ -243,6 +243,12 @@ systemd-run --wait -P \
             -p RootImagePolicy='root=signed+lol:wut=wat+signed' \
             -p MountAPIVFS=yes \
             cat /usr/lib/os-release | grep -F "MARKER=1" >/dev/null
+# A policy pinning a single fstype on a GPT image should still use verity.
+systemd-run --wait -P \
+            -p RootImage="$MINIMAL_IMAGE.gpt" \
+            -p RootImagePolicy='root=verity+squashfs' \
+            -p MountAPIVFS=yes \
+            cat /usr/lib/os-release | grep -F "MARKER=1" >/dev/null
 (! systemd-run --wait -P \
                -p RootImage="$MINIMAL_IMAGE.gpt" \
                -p RootHash="$MINIMAL_IMAGE_ROOTHASH" \