Tests: Fix 22 warning in BuildCommandTests (swiftlang#8509)

### Motivation:

Reduce noise from solvable warnings.

### Modifications:

We should `throw XCTSkip("...")` but `try XCTSkipIf(true, "...")`.
The BuildCommandTests was mixing both (i.e., `try XCTSkip("...")`), 
making the test a nop actually.

Author: pmattos