Add a regression test for trailing skippable frames

[methakon]

Refs #129

Why a test and no fix

The bug is real but already fixed on 1.x by "Keep decompressing as long as we have compressed data" (#155): that change made the streaming fallback keep consuming input after the first frame, so a trailing skippable frame no longer reads as truncated input. #129 was never closed, and nothing on 1.x covers the trailing-skippable shape — the two tests #155 added cover concatenated streams and truncated input, not this case.

What the test pins down

TestDecompressTrailingSkippableFrame covers the one-shot Decompress path with the exact shape from #129: a highly compressible 1,000,001-byte payload (so the compressed input is tiny and the streaming fallback runs), followed by a 12-byte skippable frame. That test fails with unexpected EOF on the parent of #155 and passes on current 1.x.

TestReaderTrailingSkippableFrame covers the same through the streaming reader with random data.

Verified before opening:

at #155~1:  TestDecompressTrailingSkippableFrame  FAIL  "unexpected EOF"
            TestReaderTrailingSkippableFrame      PASS
on 1.x:     both PASS
go test .   full suite ok

The go vet unsafe.Pointer warnings in zstd_stream.go and the gofmt flag on zstd_stream_test.go are pre-existing and untouched by this change.

If maintainers would rather #129 simply be closed as fixed by #155, that's fine too — happy to close this if so.