test: fixup testing for specific N-API version (#840)

Fixup testing for a specific N-API version
and document.

Reviewed-By: NickNaso <nicoladelgobbo@gmail.com>
PR-URL: nodejs/node-addon-api#840

README.md

@@ -157,6 +157,15 @@ npm install
 npm test --disable-deprecated
 ```
 
+To run the tests targetting a specific version of N-API run
+```
+npm install
+export NAPI_VERSION=X
+npm test --NAPI_VERSION=X
+```
+
+where X is the version of N-API you want to target.
+
 ### **Debug**
 
 To run the **node-addon-api** tests with `--debug` option:

test/index.js

@@ -62,7 +62,15 @@ let testModules = [
   'version_management'
 ];
 
-const napiVersion = Number(process.versions.napi)
+let napiVersion = Number(process.versions.napi)
+if (process.env.NAPI_VERSION) {
+  // we need this so that we don't try run tests that rely
+  // on methods that are not available in the NAPI_VERSION
+  // specified
+  napiVersion = process.env.NAPI_VERSION;
+}
+console.log('napiVersion:' + napiVersion);
+
 const majorNodeVersion = process.versions.node.split('.')[0]
 
 if (napiVersion < 3) {