[Test] Add back-deployed concurrency library path for runtime OS testing

When testing the runtime in the OS (via the lit parameter
`use_os_stdlib`), add the path to the back-deployed concurrency
libraries into `DYLD_LIBRARY_PATH` for the test run. This should allow
us to run our regression tests on older OS's.

Author: DougGregor

test/lit.cfg

@@ -408,7 +408,7 @@ config.swift_test_options = '-swift-version ' + swift_version
 
 # Define a macro for the next release OS version.
 # This macro can be used in code as `@available(SwiftStdlib 5.5, *)`.
-if 'back_deploy_concurrency' in lit_config.params:
+if 'back_deploy_concurrency' in config.available_features:
     swift_stdlib_macro = '\'SwiftStdlib 5.5:macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0\''
 else:
     swift_stdlib_macro = '\'SwiftStdlib 5.5:macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0\''
@@ -1913,12 +1913,17 @@ if not kIsWindows:
 	else:
 		config.available_features.add('use_os_stdlib')
 		os_stdlib_path = ''
+		concurrency_back_deploy_path = ''
 		if run_vendor == 'apple':
 			#If we get swift-in-the-OS for non-Apple platforms, add a condition here
 			os_stdlib_path = "/usr/lib/swift"
 			if run_os == 'maccatalyst':
 			    os_stdlib_path = "/System/iOSSupport/usr/lib/swift:/usr/lib/swift"
-		all_stdlib_path = os.path.pathsep.join((os_stdlib_path, target_stdlib_path))
+			if 'back_deploy_concurrency' in config.available_features:
+			  concurrency_back_deploy_path = os.path.join(os.path.dirname(swift_obj_root), os.path.basename(swift_obj_root).replace("swift-", "backdeployconcurrency-"), 'lib', 'swift-5.5', run_os)
+
+		all_stdlib_path = os.path.pathsep.join((os_stdlib_path, concurrency_back_deploy_path, target_stdlib_path))
+
 		lit_config.note('Testing with the standard libraries coming from the OS ' + all_stdlib_path)
 		config.target_run = (
 			"/usr/bin/env "