[Tests] add fixture with remapped folder

[bgotink]

Continuation of #78 which I inadvertently closed by deleting the fork repo.

---

Adds a fixture with a remapped folder, where the remapped name also exists. The example is a bit contrived, but it does showcase two bugs:

1. The /mapped/ export is mapped onto /unmapped/, making it impossible to require any of the files in the actual mapped folder. The mapped/index.js file does show up in the result. This is linked to Incorrect main export for preact listed when run without conditions #77
2. The "tree" doesn't show the mapping

In other words, I'd expect

 "tree": {
   "ex-folder-mapped": {
     "unmapped": {
       "index.js": [
+        "ex-folder-mapped/mapped",
+        "ex-folder-mapped/mapped/",
+        "ex-folder-mapped/mapped/index",
+        "ex-folder-mapped/mapped/index.js",
         "ex-folder-mapped/unmapped",
         "ex-folder-mapped/unmapped/",
         "ex-folder-mapped/unmapped/index",
         "ex-folder-mapped/unmapped/index.js"
       ]
     },
     "index.js": [
       "ex-folder-mapped",
       "ex-folder-mapped/index",
       "ex-folder-mapped/index.js"
     ],
     "package.json": [
       "ex-folder-mapped/package",
       "ex-folder-mapped/package.json"
-    ],
-    "mapped": {
-      "index.js": [
-        "ex-folder-mapped/mapped",
-        "ex-folder-mapped/mapped/",
-        "ex-folder-mapped/mapped/index",
-        "ex-folder-mapped/mapped/index.js"
-      ]
-    }
+    ]
   }
 },

[ljharb]

Thanks, got this passing :-)