Migrate deprecated eslint config to to new format

The .eslintrc and .eslintignore formats are deprecated. The new format
uses a single eslint.config.js (or .mjs) file at the top.

tests/.eslintrc

@@ -1,15 +0,0 @@
-{
-    "env": {
-        "node": true,
-        "mocha": true
-    },
-    "globals": {
-        "chai": false,
-        "sinon": false
-    },
-    "rules": {
-        "prefer-arrow-callback": 0,
-        // Too many anonymous callbacks
-        "func-names": "off",
-    }
-}