Cleaning up some testing stuff.

- The Makefiles now run btest with "-f diag.log" so that diangnostics
  output will always be written into that file.

- Makefiles now hardcode path to btest to avoid picking up the wrong version
  if in PATH.

- The canonifier scripts now live in testing/scripts, and they are
  used from both btest/ and external/.

- There's a new diff-remove-uids scripts that removed connection UIDs
  for diffing. The external/* tests now use that by default.

- Timestamp removal now has its own script: diff-remove-timestamps.
  diff-canonifier calls that.

- All Makefile have a "brief" target that runs btest with -b.

- The higher-level directories have Makefile to call the subdirs.

testing/external/scripts/skel/test.skeleton

@@ -0,0 +1,2 @@
+@TEST-EXEC: bro -r $TRACES/test.trace test-all
+@TEST-EXEC: for i in *.log; do btest-diff $i; done