[test][embedded] REQUIRE synchronization in test that uses synchornization (#72329)

This tests imports the Synchronization module, so it will not work if
Synchonization is not part of the build. Mark it as `REQUIRES:
synchronization` to skip it in case synchronization has not been built.

Author: drodriguez

test/embedded/synchronization.swift

@@ -7,6 +7,7 @@
 // REQUIRES: executable_test
 // REQUIRES: optimized_stdlib
 // REQUIRES: OS=macosx || OS=linux-gnu
+// REQUIRES: synchronization
 
 import Synchronization