Fix test inaccuracy on Darwin

[lherman-cs]

In Github Actions' Darwin environment, timing related tests are not accurate. For example, a throttle test expects the API to throttle a video's fps to go down to 30 fps. But, this inaccuracy makes the fps output to be lower than the expected 30 fps.

[at-wat]

=== RUN   TestMeasureBitRateStatic
--- PASS: TestMeasureBitRateStatic (6.06s)
=== RUN   TestMeasureBitRateDynamic
--- FAIL: TestMeasureBitRateDynamic (5.15s)
##[error]    measurement_test.go:106: expected: 25600.000000 (with 8.000000 precision), but got 24000.000000
FAIL

TestMeasure* should be skipped on Darwin as well.