test: remove unnecessary #ifdef _WIN32

This is a back-port of commit 4d17337 from the master branch.

test/test-spawn.c

@@ -388,11 +388,7 @@ TEST_IMPL(spawn_preserve_env) {
   options.stdio[1].data.stream = (uv_stream_t*) &out;
   options.stdio_count = 2;
 
-#ifdef _WIN32
   r = putenv("ENV_TEST=testval");
-#else
-  r = setenv("ENV_TEST", "testval", 1);
-#endif
   ASSERT(r == 0);
 
   /* Explicitly set options.env to NULL to test for env clobbering. */