Remount root fs as read-only in mktree.rootfs

This is better and more explicit than just assuming/documenting that.

tests/mktree.rootfs

@@ -1,11 +1,13 @@
 #!/bin/bash
 #
 # Rootfs mktree backend for use in throwaway build containers.
-# Installs RPM into / and runs the test-suite against the same.
 #
+# Installs RPM into the root filesystem and runs the test-suite against it.
 # PWD must be host-mounted for OverlayFS mounts to work.
-# The / filesystem should be read-only to prevent parallel tests from altering
-# it (online changes to an underlying filesystem are disallowed in OverlayFS).
+#
+# The root filesystem must be read-only and will be remounted as such otherwise
+# before rpmtests is executed.  This is needed because lowerdir is not allowed
+# to change when part of an OverlayFS (which rpmtests creates).
 
 source ./mktree.common
 
@@ -15,7 +17,22 @@ case $CMD in
         make_install /
     ;;
     check)
-        rpmtests "$@"
+        # Get root's current rw/ro mode
+        MODE=$(findmnt -n -o OPTIONS / | cut -d, -f1)
+
+        # Remount root as read-only if not already, exit if that fails
+        if [ "$MODE" == "rw" ]; then
+            mount -o remount,ro / || exit
+        fi
+
+        rpmtests "$@"; RC=$?
+
+        # Revert root back to read/write if that was the original mode
+        if [ "$MODE" == "rw" ]; then
+            mount -o remount,rw / || exit
+        fi
+
+        exit $RC
     ;;
     *)
         echo "Unsupported command." >&2