test: add test for Module._stat

Module._stat landed in nodejs#44537 without
a test, so this change adds one.

Signed-off-by: Darshan Sen <raisinten@gmail.com>

test/parallel/test-module-_stat.js

@@ -0,0 +1,54 @@
+'use strict';
+require('../common');
+
+// This tests Module._stat.
+
+const Module = require('module');
+const path = require('path');
+const tmpdir = require('../common/tmpdir');
+const { mkdirSync, writeFileSync } = require('fs');
+const { strictEqual } = require('assert');
+
+const directory = path.join(tmpdir.path, 'directory');
+const doesNotExist = path.join(tmpdir.path, 'does-not-exist');
+const file = path.join(tmpdir.path, 'file');
+
+tmpdir.refresh();
+writeFileSync(file, '');
+mkdirSync(directory);
+
+strictEqual(Module._stat(directory), 1);
+strictEqual(Module._stat(doesNotExist), -2);
+strictEqual(Module._stat(file), 0);
+
+const vfsDirectory = path.join(process.execPath, 'directory');
+const vfsDoesNotExist = path.join(process.execPath, 'does-not-exist');
+const vfsFile = path.join(process.execPath, 'file');
+
+strictEqual(Module._stat(vfsDirectory), -20);
+strictEqual(Module._stat(vfsDoesNotExist), -20);
+strictEqual(Module._stat(vfsFile), -20);
+
+const originalStat = Module._stat;
+Module._stat = function(filename) {
+  if (!filename.startsWith(process.execPath)) {
+    return originalStat(filename);
+  }
+
+  switch (filename) {
+    case vfsDirectory:
+      return 1;
+    case vfsDoesNotExist:
+      return -2;
+    case vfsFile:
+      return 0;
+  }
+};
+
+strictEqual(Module._stat(directory), 1);
+strictEqual(Module._stat(doesNotExist), -2);
+strictEqual(Module._stat(file), 0);
+
+strictEqual(Module._stat(vfsDirectory), 1);
+strictEqual(Module._stat(vfsDoesNotExist), -2);
+strictEqual(Module._stat(vfsFile), 0);