test: add arrow functions to test-util-inspect

Even though arrow functions and ES5 anonymous functions are technically
the same for util.js, it won't hurt to test both.

Backport-PR-URL: nodejs/node#11970
PR-URL: nodejs/node#11781
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>

test/parallel/test-util-inspect.js

@@ -9,6 +9,7 @@ assert.strictEqual(util.inspect(false), 'false');
 assert.strictEqual(util.inspect(''), "''");
 assert.strictEqual(util.inspect('hello'), "'hello'");
 assert.strictEqual(util.inspect(function() {}), '[Function]');
+assert.strictEqual(util.inspect(() => {}), '[Function]');
 assert.strictEqual(util.inspect(undefined), 'undefined');
 assert.strictEqual(util.inspect(null), 'null');
 assert.strictEqual(util.inspect(/foo(bar\n)?/gi), '/foo(bar\\n)?/gi');
@@ -28,6 +29,7 @@ assert.strictEqual(util.inspect([1, [2, 3]]), '[ 1, [ 2, 3 ] ]');
 assert.strictEqual(util.inspect({}), '{}');
 assert.strictEqual(util.inspect({a: 1}), '{ a: 1 }');
 assert.strictEqual(util.inspect({a: function() {}}), '{ a: [Function: a] }');
+assert.strictEqual(util.inspect({a: () => {}}), '{ a: [Function: a] }');
 assert.strictEqual(util.inspect({a: 1, b: 2}), '{ a: 1, b: 2 }');
 assert.strictEqual(util.inspect({'a': {}}), '{ a: {} }');
 assert.strictEqual(util.inspect({'a': {'b': 2}}), '{ a: { b: 2 } }');