Though I have never heard the specific phrasing "crossing the straits", perhaps it is referring the following definition:
Strait
[streyt]
/noun
- (Often straits) a position of difficulty, distress, or need.
Source: Dictionary.com
If this is accurate, perhaps the phrase would mean, "crossing" the threshold into a level of serious distress. Hence the command stops the tests by powering down the aircraft, rather than allowing the test to continue.
Alternately, if it may be referencing a specific geographic location being used as a landmark in the test. However, I don't believe there is any way to corroborate that, since there is neither visual indication in the movie shot, nor any reference to it elsewhere in the film.
Strait
[streyt]
/noun
- (Often straits, used with a singular verb) a narrow passage of water connecting two large bodies of water.
Source: Dictionary.com