Reduce the number of OpenGL and EGL tests to 16, to avoid exceeding the
maximum number of X clients.

Also make the attempts to open an X display back off exponentially
instead of linearly.

Bug: b/153322216
Change-Id: Ifb91d9b892dc4b5f6d26f369f2bece454b0bae87
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48969Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>