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`.