compress: move benchmark text from src/testdata to src/compress/testdata

This text is used mainly for benchmark compression testing, and in one
net test. The text was prevoiusly in a src/testdata directory, but since
that directory would only include one file, the text is moved to the
existing src/compression/testdata directory.

This does not cause any change to the benchmark results.

Updates #27151

Change-Id: I38ab5089dfe744189a970947d15be50ef1d48517
Reviewed-on: https://go-review.googlesource.com/138495


Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>