Relax qunit peer dependency minimum version to ^2.13.0.

[email protected] shipped with this peer dependency set to at least
2.13.0. When Dependabot landed a dependency update for our internal
QUnit dev dependency it also increased the minimum for the peerDep (in
719af61)
which was released in [email protected].

Changing the minimum version of a peer dependency that we enforce (via
`validatePeerDependencies`) should be considered a breaking change!

This reverts that change and sets the qunit peer dep back to its value
when 5.0.0 was published.

package.json

@@ -69,7 +69,7 @@
   },
   "peerDependencies": {
     "@ember/test-helpers": "^2.2.3",
-    "qunit": "^2.14.0"
+    "qunit": "^2.13.0"
   },
   "engines": {
     "node": "10.* || 12.* || >= 14.*"