add SWIFT_BUILD_TESTS option for building Swift test packages

Add a SWIFT_BUILD_TESTS option to enable building tests, defaults to
DEBUG_BUILD.

Author: lhoward

classes/swift.bbclass

@@ -1,7 +1,10 @@
 # avoid conflicts with meta-clang
 TOOLCHAIN = "gcc"
 
-DEPENDS += "swift-native glibc gcc libgcc swift-stdlib libdispatch libfoundation"
+SWIFT_BUILD_TESTS ?= "${DEBUG_BUILD}"
+
+DEPENDS += "swift-native glibc gcc libgcc swift-stdlib libdispatch swift-foundation"
+DEPENDS += "${@oe.utils.conditional('SWIFT_BUILD_TESTS', '1', 'swift-xctest swift-testing', '', d)}"
 
 # Default build directory for SPM is "./.build"
 # (see 'swift [build|package|run|test] --help')