remove TestExt.jl

[nhz2]

Fixes #223 Fixes #234
Removes:

function test_roundtrip_read end
function test_roundtrip_write end
function test_roundtrip_transcode end
function test_roundtrip_lines end
function test_roundtrip_seekstart end
function test_roundtrip_fileio end
function test_chunked_read end
function test_chunked_write end

These functions have been moved into lib/TestsForCodecPackages in #235

I decided not to keep the functions as noops that always pass, because:

1. I have already updated all usage of the functions I could find on JuliaHub. Ref: use TestsForCodecPackages.jl CodecZlib.jl#84 use TestsForCodecPackages.jl CodecInflate64.jl#2 use TestsForCodecPackages CodecXz.jl#29 use TestsForCodecPackages.jl CodecBase.jl#17 use TestsForCodecPackages.jl CodecZstd.jl#61 use TestsForCodecPackages.jl CodecBzip2.jl#34 use TestsForCodecPackages.jl CodecLz4.jl#58
2. These functions are not exported or documented, so they are not part of the API of this package.
3. It is better for a no longer working testing function to fail loudly.