test: avoid copying test source files

Converting the helper functions to be inlined and making the helper file
header only.

PR-URL: https://github.com/nodejs/node/pull/49515
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gabriel Schulhof <gabrielschulhof@gmail.com>

test/js-native-api/test_date/binding.gyp

@@ -3,7 +3,6 @@
     {
       "target_name": "test_date",
       "sources": [
-        "../entry_point.c",
         "test_date.c"
       ]
     }

test/js-native-api/test_date/test_date.c

@@ -1,5 +1,6 @@
 #include <js_native_api.h>
 #include "../common.h"
+#include "../entry_point.h"
 
 static napi_value createDate(napi_env env, napi_callback_info info) {
   size_t argc = 1;