binary: improve tests for invalid numbers

[kytrinyx]

We should have separate tests for:

• alphabetic characters at the beginning of a valid binary number
• alphabetic characters at the end of a valid binary number
• alphabetic characters in the middle of an otherwise valid binary number
• invalid digits (e.g. 2)

If the test suite for binary has test cases that cover these edge cases, this issue can safely be closed.

See exercism/problem-specifications#95

[matthewmorgan]

This track is missing a few of the above mentioned tests. It does have a test that puts alphabetic characters at the beginning AND end of the input (in the same single test). To make it complete we should add:

• alphabetic characters at the beginning of a valid binary number
• alphabetic characters at the end of a valid binary number
• alphabetic characters in the middle of an otherwise valid binary number

[kytrinyx]

Yeah, you could probably split the existing test into two tests (I think if you have one for the beginning and one for the end, a test for both beginning and end is redundant).

[kytrinyx]

I'm going to go ahead and close this, due to exercism/problem-specifications#279