This had regressed mostly due to a single thread processing a huge number of `Span.Add()`s.

Optimize for the common case, avoid `Span.Add()` in places where the data is already sorted, don't scan the entire file system for every test run.

Bug: b/152192800
Change-Id: Id183468263e65bfbcf387ec1c978d8b9de547cee
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43570Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>