Add a .coffee test proving extension resolve order

Add a test for a more unconventional extension to prove that the
"extensions" rule can take anything, as long as the extension resolve
order is correct, i.e. it can only remove the extension if it would
resolve to the correct file.

Ref. #965.
Closes #583.

tests/files/bar.coffee

@@ -0,0 +1 @@
+console.log 'bar'

tests/src/rules/extensions.js

@@ -183,7 +183,7 @@ ruleTester.run('extensions', rule, {
         },
       ],
     }),
-    // extension resolve order (#965)
+    // extension resolve order (#583/#965)
     test({
       code: [
         'import component from "./bar.jsx"',
@@ -199,6 +199,18 @@ ruleTester.run('extensions', rule, {
         },
       ],
     }),
+    test({
+      code: 'import "./bar.coffee"',
+      errors: [
+        {
+          message: 'Unexpected use of file extension "coffee" for "./bar.coffee"',
+          line: 1,
+          column: 8,
+        },
+      ],
+      options: ['never', { js: 'always', jsx: 'always' }],
+      settings: { 'import/resolve': { 'extensions': ['.coffee', '.js'] } },
+    }),
 
     test({
       code: [