tests: move mock blackbox extension into own file
Having all blackbox log testing in test-blackbox.t isn't scalable. Move
the mock blackbox extension into its own file so we can start to move
blackbox logging into other tests.
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..