test: add get/set effective uid/gid tests

3c92ca2 should have had tests
to go along with it. This adds tests for the following functions:

* `process.geteuid()`
* `process.seteuid()`
* `process.getegid()`
* `process.setegid()`

PR-URL: #14091
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-process-geteuid-getegid.js

@@ -0,0 +1,49 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+
+if (common.isWindows) {
+  assert.strictEqual(process.geteuid, undefined);
+  assert.strictEqual(process.getegid, undefined);
+  assert.strictEqual(process.seteuid, undefined);
+  assert.strictEqual(process.setegid, undefined);
+  return;
+}
+
+assert.throws(() => {
+  process.seteuid({});
+}, /^TypeError: seteuid argument must be a number or string$/);
+
+assert.throws(() => {
+  process.seteuid('fhqwhgadshgnsdhjsdbkhsdabkfabkveybvf');
+}, /^Error: seteuid user id does not exist$/);
+
+// If we're not running as super user...
+if (process.getuid() !== 0) {
+  assert.doesNotThrow(() => {
+    process.getegid();
+    process.geteuid();
+  });
+
+  assert.throws(() => {
+    process.setegid('nobody');
+  }, /^Error: (?:EPERM, .+|setegid group id does not exist)$/);
+
+  assert.throws(() => {
+    process.seteuid('nobody');
+  }, /^Error: (?:EPERM, .+|seteuid user id does not exist)$/);
+
+  return;
+}
+
+// If we are running as super user...
+const oldgid = process.getegid();
+process.setegid('nobody');
+const newgid = process.getegid();
+assert.notStrictEqual(newgid, oldgid);
+
+const olduid = process.geteuid();
+process.seteuid('nobody');
+const newuid = process.geteuid();
+assert.notStrictEqual(newuid, olduid);