[tests] Add support for probes.json (#8279)

Previously, our test fixtures used to use a probes prop in `vercel.json` that was removed right before it was deployed.

This PR allows a separate `probes.json` file with the same content to separate the test fixture input from the test probes.

This allows us to test real "zero config" deployments without a `vercel.json` file.

packages/static-build/test/fixtures/75-pnpm7-custom-commands/probes.json

@@ -0,0 +1,8 @@
+{
+  "probes": [
+    {
+      "path": "/",
+      "mustContain": "pnpm version: 7"
+    }
+  ]
+}

packages/static-build/test/fixtures/75-pnpm7-custom-commands/vercel.json

@@ -1,15 +0,0 @@
-{
-  "version": 2,
-  "builds": [
-    {
-      "src": "package.json",
-      "use": "@vercel/static-build"
-    }
-  ],
-  "probes": [
-    {
-      "path": "/",
-      "mustContain": "pnpm version: 7"
-    }
-  ]
-}