Fix #5435, adding new test config check_stdout.

check_stdout(f) allows you to override the test framework's
diff based output checking with another mechanism.  f is
a function which takes two arguments: the first is the
filename containing the observed stdout, the second is the
normaliser that would have been applied (in case you want
to read, normalise, and then do something.)

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>