Only use the condition in the `endif()` if:
• The opening `if()` is more than 20 lines away.
• The `endif()` doesn't belong to an `elif()`.

Also remove redundant sequences of `endif(SWIFTSHADER_BUILD_TESTS)` `if(SWIFTSHADER_BUILD_TESTS)`

Bug: None. Style bikeshedding
Change-Id: I967948a2afdedbd503add5addffc666c698eb885
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40468
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>