Reduce number of threads and iterations.
These tests were taking a long time.

Should around ~16x faster.

Bug: b/153803432
Change-Id: I3b0e6753852e7bb3a4ed44359e7d8446fd4120eb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44408
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>