[test] Move embedded wasm tests to a separate directory

The `classes-wasm.swift` test was the only executable test for
WebAssembly that requires wasm runtime at test-time. Other tests
in the `embedded` directory run only on the macOS / Linux host toolchain
builds and some of them are incompatible with SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB=OFF.
Given that the `classes-wasm.swift` test is the only test we want to run
during WasmStdlib build, move it to a separate directory `embedded/wasm`.

Author: kateinoigakukun

test/embedded/classes-wasm.swift renamed to test/embedded/wasm/classes.swift

@@ -114,7 +114,7 @@ def _build(self, host_target, target_triple):
         # Test configuration
         self.cmake_options.define('SWIFT_INCLUDE_TESTS:BOOL', 'TRUE')
         self.cmake_options.define('SWIFT_ENABLE_SOURCEKIT_TESTS:BOOL', 'FALSE')
-        lit_test_paths = ['IRGen', 'stdlib', 'Concurrency/Runtime', 'embedded']
+        lit_test_paths = ['IRGen', 'stdlib', 'Concurrency/Runtime', 'embedded/wasm']
         lit_test_paths = [os.path.join(
             self.build_dir, 'test-wasi-wasm32', path) for path in lit_test_paths]
         self.cmake_options.define('SWIFT_LIT_TEST_PATHS:STRING',