BUG: gap_detection: added to Allrun

tutorials/mesh/snappyHexMesh/Allrun

@@ -12,6 +12,11 @@ cd ${0%/*} || exit 1    # Run from this directory
     ./Allrun
 )
 
+(
+    cd gap_detection || exit
+    ./Allrun
+)
+
 exit 0
 
 # These cases are links to solver test cases and are run when the Allrun