Rather than load/unload the driver for each test run, only do so once
per suite. This is mainly a workaround for the memory leak bug
reported at b/173733181; however, it also speeds up running the tests.

Bug: b/173733181
Change-Id: I7d47f8eee2fe8010bbf06845877649abf6761256
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50608
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>