Clean out localdir before tests

Shows up when there are no deps and lots of files can be 'left'.

l3build.lua

@@ -839,6 +839,7 @@ end
 -- for saving the test files
 function checkinit()
   cleandir(testdir)
+  cleandir(localdir)
   depinstall(checkdeps)
   -- Copy dependencies to the test directory itself: this makes the paths
   -- a lot easier to manage, and is important for dealing with the log and