SF: libsurfaceflinger_unittest should skip SF ctor

This reduces the cost of constructing a SurfaceFlinger instance for each
test, but it means the tests must take care to set up all internal state
that is needed for each test.

[Test execution time reduced from 6.1 to 3.1ms]

Test: libsurfaceflinger_unittest passes on Pixel XL
Bug: 74827900
Change-Id: I7eed4fd274cae521fe0f22b655cf3343cee66342