Run test in well-defined directory (#51)

diff --git a/spec/fetch-unit.spec.js b/spec/fetch-unit.spec.js
index 889ef23..9cbe08b 100644
--- a/spec/fetch-unit.spec.js
+++ b/spec/fetch-unit.spec.js
@@ -119,7 +119,7 @@
     });
 
     it('should not find a package installed elsewhere', () => {
-        return resolvePathToPackage('dummy-local-plugin').then(
+        return resolvePathToPackage('dummy-local-plugin', tmpDir).then(
             _ => fail('expect promise to be rejected'),
             err => expect(err).toBeDefined()
         );