And add test to check it behaves as expected.

While currently nothing golang actually parses these files, this allows me to experiment with compression schemes on the full data set without needing to rebuild the coverage data from scratch each time.

Bug: b/152192800
Change-Id: I8070abe2bbebaef3e5889991060f1d1342078eb2
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43248
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>