Missing test cases

Already among the first few solutions I looked at, I found some mistakes missed by the current test cases.

This fails `longest_prefix(['aa', 'ba'])`, returning `'a'` instead of `''`.

This fails `longest_prefix(['a', 'ba'])`, returning `'a'` instead of `''`.

This fails `longest_prefix(['aa', 'ba'])`, returning `'a'` instead of `''`.

Btw, the initial code contains this example call:

print(longest_prefix(["flower", "flow", "flight"]))

When I modify the array so there's no common prefix, it looks like nothing gets printed. I suggest to add `repr` for clarity, so `''` gets printed.

print(repr(longest_prefix(["flower", "flow", "flight"])))

And now I'm hoping all my markdown works... It doesn't in the preview, code parts aren't styled as code... (Edit: I see... Backticks aren't supported at all, and only code blocks are supported by indenting, inline code isn't supported at all? Sigh.)

@freeman_lex