[Test] Disabled two PlaygroundTransform tests for back_deployment_runtime.

rdar://76567375

Author: nate-chandler

test/PlaygroundTransform/implicit_return_never.swift

@@ -7,6 +7,7 @@
 
 // The runtime error format changed after the 5.3 release.
 // UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
 
 // NOTE: not.py is used above instead of "not --crash" because simctl's exit
 // status doesn't reflect whether its child process crashed or not. So "not

test/PlaygroundTransform/placeholder.swift

@@ -8,6 +8,7 @@
 
 // The runtime error format changed after the 5.3 release.
 // UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
 
 // NOTE: not.py is used above instead of "not --crash" because simctl's exit
 // status doesn't reflect whether its child process crashed or not. So "not