Add NODE_PATH check and warning to tests runner

See: TypeStrong#1697 (comment)

ava.config.cjs

@@ -24,22 +24,40 @@ module.exports = {
    * ts-node, from either node_modules or tests/node_modules.
    * 
    * Another possibility of interference is NODE_PATH environment variable being set,
-   * and ts-node being installed in any of the paths listed in NODE_PATH, to fix this,
+   * and ts-node being installed in any of the paths listed on NODE_PATH, to fix this,
    * the NODE_PATH variable must be removed from the environment *BEFORE* running ava.
    * An error will be thrown when trying to run tests with NODE_PATH set to paths with ts-node installed.
    */
 
-  const { existsSync } = require('fs');
+  const { existsSync, realpathSync } = require('fs');
   const rimraf = require('rimraf');
-  const { resolve } = require('path');
+  const { resolve, delimiter } = require('path');
 
   remove(resolve(__dirname, 'node_modules/ts-node'));
   remove(resolve(__dirname, 'tests/node_modules/ts-node'));
 
   // Prove that we did it correctly
-  expect(() => {createRequire(resolve(__dirname, 'tests/foo.js')).resolve('ts-node')}).toThrow();
+  let resolved;
+  try {
+    expect(() => { resolved = createRequire(resolve(__dirname, 'tests/foo.js')).resolve('ts-node'); }).toThrow();
+  } catch (err) {
+    err.message = err.message.split('\n').slice(0, -1).join('\n') + `\nts-node unexpectedly resolved to external location: ${resolved}`;
+
+    // Check for NODE_PATH interference. See comment on line 26.
+    if (process.env.NODE_PATH) {
+      const NODE_PATH = process.env.NODE_PATH.split(delimiter).filter(f => f).map(f => realpathSync(resolve(__dirname, f)));
+      for (const path of NODE_PATH) {
+        if (resolved.includes(path)) {
+          err.message += `\n! WARNING: NODE_PATH is set and contains ts-node at ${path}`;
+          err.message += `\n! This can cause problems with tests. Please clear the NODE_PATH environment variable before running tests.\n`;
+        }
+      }
+    }
+
+    throw err;
+  }
 
   function remove(p) {
-    if(existsSync(p)) rimraf.sync(p, {recursive: true})
+    if (existsSync(p)) rimraf.sync(p, {recursive: true});
   }
 }