Replace test using CLIEngine class with ESLint class. (#299)

* Replace test using CLIEngine class with ESLint class.

This change is a preparation for ESLint v8 support.

* fix

Author: ota-meshi

tests/lib/eslint-plugin.ts

@@ -1,6 +1,6 @@
 import path from "path"
 import assert from "assert"
-import { CLIEngine } from "eslint"
+import { ESLint } from "eslint"
 import plugin from "../../lib/index"
 
 // -----------------------------------------------------------------------------
@@ -10,16 +10,26 @@ import plugin from "../../lib/index"
 const TEST_CWD = path.join(__dirname, "../fixtures/integrations/eslint-plugin")
 
 describe("Integration with eslint-plugin-regexp", () => {
-    it("should lint without errors", () => {
-        const engine = new CLIEngine({
-            cwd: TEST_CWD,
-        })
-        engine.addPlugin("eslint-plugin-regexp", plugin)
-        const r = engine.executeOnFiles(["test.js"])
+    it("should lint without errors", async () => {
+        let results: ESLint.LintResult[]
+        if (ESLint) {
+            const eslint = new ESLint({
+                cwd: TEST_CWD,
+                plugins: { "eslint-plugin-regexp": plugin },
+            })
+            results = await eslint.lintFiles(["test.js"])
+        } else {
+            // eslint-disable-next-line @typescript-eslint/no-var-requires, @typescript-eslint/no-require-imports -- ignore
+            const engine = new (require("eslint").CLIEngine)({
+                cwd: TEST_CWD,
+            })
+            engine.addPlugin("eslint-plugin-regexp", plugin)
+            results = engine.executeOnFiles(["test.js"]).results
+        }
 
-        assert.strictEqual(r.results.length, 1)
+        assert.strictEqual(results.length, 1)
         assert.deepStrictEqual(
-            r.results[0].messages.map((m) => m.ruleId),
+            results[0].messages.map((m) => m.ruleId),
             [
                 "regexp/no-dupe-characters-character-class",
                 "regexp/prefer-w",