wasi-runtimes: improve test

This matches what upstream are doing in their own tests.[^1]

[^1]: See https://github.com/WebAssembly/wasi-sdk/blob/a4d918fa119c9beb712bf08bbb8fa9996aab0a71/tests/CMakeLists.txt#L81-L84

Formula/w/wasi-runtimes.rb

@@ -209,7 +209,7 @@ def install
       system clang, "--target=#{target}", "-v", "test.c", "-o", "test-#{target}"
       assert_equal "the answer is 42", shell_output("wasmtime #{testpath}/test-#{target}")
 
-      pthread_flags = target.end_with?("-threads") ? ["-D_WASI_EMULATED_PTHREAD"] : []
+      pthread_flags = target.end_with?("-threads") ? ["-pthread"] : []
       system "#{clang}++", "--target=#{target}", "-v", "test.cc", "-o", "test-cxx-#{target}", *pthread_flags
       assert_equal "hello from C++ main with cout!", shell_output("wasmtime #{testpath}/test-cxx-#{target}").chomp
     end