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

[NobbZ]

I'd prefer to have a JSON with testdata before fixing this. Fixing without might introduce additional noise when there appears common testdata.

[NobbZ]

Since we are in the process of deprecating binary along other exercises in a big sweep (exercism/problem-specifications#279), I think we can close this, can't we?

[kytrinyx]

agreed, thanks!