Tests: remove @testable import DriverSupport

Use the newly extended APIs for testing and use a non-testable import of
the package, avoiding a recompilation.

Author: compnerd

Tests/BuildTests/BuildPlanTests.swift

@@ -13,7 +13,6 @@
 import Basics
 import Build
 
-@testable
 @_spi(SwiftPMInternal)
 import DriverSupport