AddTarget incorrectly assumes an XCTest dependency.

[grynspan]

The new AddTarget code should not always assume that XCTest is a dependency for a new test target. Rather, behaviour should be configured based on the --enable-xctest/--disable-xctest and --enable-experimental-swift-testing/--disable-experimental-swift-testing flags, with XCTest on and swift-testing off by default. Compare the swift package init command.