Update tests to treat multithreaded_present as true

These tests were written or updated as if the multithreaded_present
flag is true, but may run on devices where it is not. Explicitly set the
flag to true so the tests always pass.

Bug: 323232586
Test: libsurfaceflinger_unittest
Change-Id: Ica7a921b5bf01b8eadac78900ac69f854f8a02de