test: increase fs.exists coverage

PR-URL: nodejs/node#15963
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/parallel/test-fs-exists.js

@@ -30,6 +30,8 @@ fs.exists(f, common.mustCall(function(y) {
   assert.strictEqual(y, true);
 }));
 
+assert.doesNotThrow(() => fs.exists(f));
+
 fs.exists(`${f}-NO`, common.mustCall(function(y) {
   assert.strictEqual(y, false);
 }));