Add relative path cache spec

spec/fixtures/module-cache/file.json

@@ -0,0 +1,3 @@
+{
+  "foo": "bar"
+}

spec/module-cache-spec.coffee

@@ -1,5 +1,6 @@
 path = require 'path'
 Module = require 'module'
+ModuleCache = require '../src/module-cache'
 
 describe 'ModuleCache', ->
   beforeEach ->
@@ -8,3 +9,14 @@ describe 'ModuleCache', ->
   it 'resolves atom shell module paths without hitting the filesystem', ->
     require.resolve('shell')
     expect(Module._findPath.callCount).toBe 0
+
+  it 'resolves relative core paths without hitting the filesystem', ->
+    ModuleCache.add atom.getLoadSettings().resourcePath, {
+      _atomModuleCache:
+        extensions:
+          '.json': [
+            path.join('spec', 'fixtures', 'module-cache', 'file.json')
+          ]
+    }
+    expect(require('./fixtures/module-cache/file.json').foo).toBe 'bar'
+    expect(Module._findPath.callCount).toBe 0