Refactoring the test suite I found that the old etap was only as strict
as the `==` operator where as EUnit uses the `=:=` operator.

I pulled in a lot of tests from the double-conversion test suite to make
sure we don't accidentally break behavior in the future.

Rebar's eunit command wants to try and compile anything named *.erl in
the tests/ directory which causes errors with the test expectation
files. Rather than write a patch for rebar it was easier to just rename
everything.

The encoder can now return \u escaped unicode data instead of leaving
it as UTF-8 byte sequences. This done like so:

    Eshell V5.8.3  (abort with ^G)
    1> jiffy:encode(<<240, 144, 129, 128>>, [uescape]).
    <<"\"\\uD800\\uDC40\"">>

Passes all eep0018 tests.