test: make TEST-86-MULTI-PROFILE-UKI robust against reruns

When qemu reboots instead of shutting down after the last iteration,
the profile is already set to profile2 but the /root/encrypted.raw is
gone so the test fails. Reset the default boot entry at the end of the
test to make it robust against reruns.

Fixes https://github.com/systemd/systemd/issues/39553

test/units/TEST-86-MULTI-PROFILE-UKI.sh

@@ -64,6 +64,9 @@ elif [[ "$ID" == "profile1" ]]; then
 elif [[ "$ID" == "profile2" ]]; then
     grep testprofile2=1 /proc/cmdline
     rm /root/encrypted.raw
+    # Reset the default boot entry so a subsequent re-run of the test does not
+    # boot straight back into @profile2 (where encrypted.raw is now gone) and fail.
+    bootctl set-default ""
 else
     exit 1
 fi