fix(gatsby-plugin-mdx): Fix not passing file path to remark plugins (g…

…atsbyjs#26914)

* Fix gatsby-plugin-mdx not passing file path to remark plugins

* Add comments for the test

yarn.lock

@@ -1922,7 +1922,12 @@
     "@types/node" "*"
     form-data "^3.0.0"
 
-"@types/node@*", "@types/node@^8.5.7":
+"@types/node@*":
+  version "14.10.2"
+  resolved "https://registry.yarnpkg.com/@types/node/-/node-14.10.2.tgz#9b47a2c8e4dabd4db73b57e750b24af689600514"
+  integrity sha512-IzMhbDYCpv26pC2wboJ4MMOa9GKtjplXfcAqrMeNJpUUwpM/2ATt2w1JPUXwS6spu856TvKZL2AOmeU2rAxskw==
+
+"@types/node@^8.5.7":
   version "8.10.64"
   resolved "https://registry.yarnpkg.com/@types/node/-/node-8.10.64.tgz#0dddc4c53ca4819a32b7478232d8b446ca90e1c6"
   integrity sha512-/EwBIb+imu8Qi/A3NF9sJ9iuKo7yV+pryqjmeRqaU0C4wBAOhas5mdvoYeJ5PCKrh6thRSJHdoasFqh3BQGILA==