[BuildSystemTaskTests] Make sure first build system is closed before building with the second one
[BuildSystemTaskTests] Make sure first build system is closed before
building with the second one

Otherwise, we might run into concurrent access on the build database.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild