toDisplayRedbox(): replace all occurrences of testDir (#76618)

Currently we only replace the first occurrence with
`String.prototype.replace()`. This uses `replaceAll` to replace all
occurrences.
    
Test Plan: See next PR in this stack for use.

test/lib/add-redbox-matchers.ts

@@ -136,7 +136,7 @@ async function createRedboxSnapshot(
     focusedSource = focusedSource.trim()
 
     if (next !== null) {
-      focusedSource = focusedSource.replace(
+      focusedSource = focusedSource.replaceAll(
         next.testDir,
         '<FIXME-project-root>'
       )