fix: accept package manager from test result if returned

When testing a GitHub url without an explicit target file
the CLI won't know which package manager was used to perform
the test, so accept the package manager from the test response
if returned.

lib/snyk-test/index.js

@@ -29,7 +29,9 @@ function executeTest(root, options) {
     options.packageManager = packageManager;
     return run(root, options)
     .then(function (res) {
-      res.packageManager = packageManager;
+      if (!res.packageManager) {
+        res.packageManager = packageManager;
+      }
       return res;
     });
   } catch (error) {