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

[dalexj]

we currently have:

  @tag :pending
  test "invalid binary is decimal 0" do
    assert Binary.to_decimal("carrot") == 0
  end

  @tag :pending
  test "invalid binary is decimal 0 II" do
    assert Binary.to_decimal("convert01") == 0
  end

  @tag :pending
  test "invalid binary is decimal 0 III" do
    assert Binary.to_decimal("10convert") == 0
  end

  @tag :pending
  test "22 is not a binary number" do
    assert Binary.to_decimal("22") == 0
  end

looks like this is all in place, unless "carrot" should be replaced by "1carrot0"

[kytrinyx]

I think 1carrot0 should come in addition to carrot.