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]

Of the items listed above, this track lacks a test for:

alphabetic characters in the middle of an otherwise valid binary number.

[matthewmorgan]

Closed by #217