remove test numbers and improve diff explanation.

[tangentstorm]

Given the following learntris...

#!/usr/bin/env python
print('x')

We get:

Running test 1: io.q
q : quit
---- sending commands ----
q
---- awaiting results ----
: The 'q' command instructs learntris to quit.
:
: Learntris should not produce any output unless
: explicitly instructed to do so.
---- expected results ----

Test 1 failed: output mismatch:
- x

There are several cleanups that should be made:

1. The lines Running test 1: io.q and Test 1 failed: output mismatch: should be modified so that both refer to the test name and neither refers to the number.
2. There should be an explanation of the diff output.

Here is an example of how an improved version might look:

Running test: [io.q]
q : quit
+--- sending commands ----
q
+--- awaiting results ----
: The 'q' command instructs learntris to quit.
:
: Learntris should not produce any output unless
: explicitly instructed to do so.
+--- expected results ----
+--- output mismatch! ----
:
: Test [io.q] failed.
:
: The following diff show the changes required
: to pass the test.
:
:  '-' indicates a line to be removed,
:  '+' indicates a line to be added.
+-------------------------
- x

[tangentstorm]

Fixed in #60 ... The diff explanation doesn't need to be in the output, but should still be in the docs. See #14