test: add missing test case

This adds coverage for CoffeeScript extension inference. The feature was
already supported, but was not tested properly.

Author: nfischer

test/shjs.js

@@ -47,13 +47,20 @@ test('CoffeeScript', t => {
   t.falsy(result.stderr);
 });
 
-test('Extension detection', t => {
+test('JS extension inference', t => {
   const result = runWithShjs('a-file');
   t.is(result.code, 0);
   t.is(result.stdout, 'OK!\n');
   t.falsy(result.stderr);
 });
 
+test('CoffeeScript extension inference', t => {
+  const result = runWithShjs('coffeescript');
+  t.is(result.code, 0);
+  t.is(result.stdout, 'CoffeeScript: OK!\n');
+  t.falsy(result.stderr);
+});
+
 //
 // Invalids
 //