t/test-emails: invoke $MULTIMAIL with fixed $USER

Since the output of $MULTIMAIL depends on $USER, set it to a temporary
constant value before invoking the command.  This will make it easier to
compare expected output with actual output.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
t/smudge-compare: add a script to smudge and compare

Prepare a multimail.expect to show what the actual output from
test-emails should look like after smudging the headers.  Introduce a
quick-and-dirty shell script called smudge-compare that compares the
actual output (after smudging) to multimail.expect.

Usage:

  $ ./test-emails >/tmp/multimail.actual 2>&1
  $ ./smudge-compare /tmp/multimail.actual

It will print "Pass" if the changes are fine.

While at it, remove the earlier t/filter-noise, and update t/README with
new instructions.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>