swiftlang/swift-driver#1741 fixed a bug in the swift-driver which
exposed a bug in this test. The use of `%FileCheck` is a problem as that
sanitises the output substituting `BUILD_DIR` for the build directory
that is now properly computed. Because `PATH` is being configured to an
invalid path the actual driver is not found and the test fails.
This test creates a shebang (#!) line that may exceed 80 characters, which is
a limit on many systems. Instead, use /usr/bin/env to invoke the right 'swift'.
OS X is more permissive (up to PATH_MAX characters), so continue testing the
direct invocation there.
