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.

tests/mockblackbox.py

@@ -0,0 +1,11 b''
+from mercurial import util
+
+def makedate():
+    return 0, 0
+def getuser():
+    return 'bob'
+
+# mock the date and user apis so the output is always the same
+def uisetup(ui):
+    util.makedate = makedate
+    util.getuser = getuser

tests/test-blackbox.t

@@ -1,20 +1,8 b''
 setup
-  $ cat > mock.py <<EOF
-  > from mercurial import util
-  > 
-  > def makedate():
-  >     return 0, 0
-  > def getuser():
-  >     return 'bob'
-  > # mock the date and user apis so the output is always the same
-  > def uisetup(ui):
-  >     util.makedate = makedate
-  >     util.getuser = getuser
-  > EOF
   $ cat >> $HGRCPATH <<EOF
   > [extensions]
   > blackbox=
-  > mock=`pwd`/mock.py
+  > mock=$TESTDIR/mockblackbox.py
   > mq=
   > EOF
   $ hg init blackboxtest