fix: handle absolute file paths in FlatRuleTester (#18064)

lib/rule-tester/flat-rule-tester.js

@@ -13,6 +13,7 @@
 const
     assert = require("assert"),
     util = require("util"),
+    path = require("path"),
     equal = require("fast-deep-equal"),
     Traverser = require("../shared/traverser"),
     { getRuleOptionsSchema } = require("../config/flat-config-helpers"),
@@ -592,7 +593,15 @@ class FlatRuleTester {
          * @private
          */
         function runRuleForItem(item) {
-            const configs = new FlatConfigArray(testerConfig, { baseConfig });
+            const flatConfigArrayOptions = {
+                baseConfig
+            };
+
+            if (item.filename) {
+                flatConfigArrayOptions.basePath = path.parse(item.filename).root;
+            }
+
+            const configs = new FlatConfigArray(testerConfig, flatConfigArrayOptions);
 
             /*
              * Modify the returned config so that the parser is wrapped to catch

tests/lib/rule-tester/flat-rule-tester.js

@@ -1808,6 +1808,22 @@ describe("FlatRuleTester", () => {
         }, /Fixable rules must set the `meta\.fixable` property/u);
     });
 
+    // https://github.com/eslint/eslint/issues/17962
+    it("should not throw an error in case of absolute paths", () => {
+        ruleTester.run("no-eval", require("../../fixtures/testers/rule-tester/no-eval"), {
+            valid: [
+                "Eval(foo)"
+            ],
+            invalid: [
+                {
+                    code: "eval(foo)",
+                    filename: "/an-absolute-path/foo.js",
+                    errors: [{ message: "eval sucks.", type: "CallExpression" }]
+                }
+            ]
+        });
+    });
+
     describe("suggestions", () => {
         it("should pass with valid suggestions (tested using desc)", () => {
             ruleTester.run("suggestions-basic", require("../../fixtures/testers/rule-tester/suggestions").basic, {