Improve logic for detecting when to use flat config format

- Don't try to use flat config format on eslint versions that don't support it
- Respect `process.env.ESLINT_USE_FLAT_CONFIG`

integrationTests/__fixtures__/legacy-config-at-eslint.config.js/__eslint__/file.js

@@ -0,0 +1,3 @@
+const a = 1;
+
+console.log('a', a);

integrationTests/__fixtures__/legacy-config-at-eslint.config.js/eslint.config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  rules: {
+    'no-console': 'error',
+  },
+};

integrationTests/__fixtures__/legacy-config-at-eslint.config.js/jest-runner-eslint.config.js

@@ -0,0 +1,3 @@
+module.exports = {
+  config: './eslint.config.js',
+};

integrationTests/__fixtures__/legacy-config-at-eslint.config.js/jest.config.js

@@ -0,0 +1,4 @@
+module.exports = {
+  runner: '../../../',
+  testMatch: ['**/__eslint__/**/*.js'],
+};

integrationTests/__snapshots__/legacy-config-at-eslint.config.js.test.js.snap

@@ -0,0 +1,22 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Does not try to use flat config on eslint versions that don't support it 1`] = `
+"PASS __eslint__/file.js
+  ● Console
+
+  console.warn
+    
+    /mocked-path-to-jest-runner-mocha/integrationTests/__fixtures__/legacy-config-at-eslint.config.js/__eslint__/file.js
+      3:1  warning  Unexpected console statement  no-console
+    
+    ✖ 1 problem (0 errors, 1 warning)
+
+  ✓ ESLint 
+
+Test Suites: 1 passed, 1 total
+Tests:       1 passed, 1 total
+Snapshots:   0 total
+Time:        
+Ran all test suites.
+"
+`;

integrationTests/legacy-config-at-eslint.config.js.test.js

@@ -0,0 +1,12 @@
+const { version } = require('eslint/package.json');
+const semver = require('semver');
+const runJest = require('./runJest');
+
+(semver.satisfies(version, '<8') ? it : it.skip)(
+  "Does not try to use flat config on eslint versions that don't support it",
+  async () => {
+    expect(
+      await runJest('legacy-config-at-eslint.config.js'),
+    ).toMatchSnapshot();
+  },
+);

package.json

@@ -28,7 +28,8 @@
     "cosmiconfig": "^7.0.0",
     "create-jest-runner": "^0.11.2",
     "dot-prop": "^6.0.1",
-    "find-up": "^5.0.0"
+    "find-up": "^5.0.0",
+    "semver": "^7.3.8"
   },
   "devDependencies": {
     "@babel/cli": "^7.10.4",
@@ -47,8 +48,7 @@
     "jest-watch-select-projects": "^2.0.0",
     "jest-watch-typeahead": "^1.1.0 || ^2.1.1",
     "prettier": "^2.8.4",
-    "rimraf": "^3.0.2",
-    "semver": "^7.3.8"
+    "rimraf": "^3.0.2"
   },
   "peerDependencies": {
     "eslint": "^7 || ^8",

src/runner/runESLint.js

@@ -1,4 +1,6 @@
 const { ESLint } = require('eslint');
+const { version } = require('eslint/package.json');
+const semver = require('semver');
 const findUp = require('find-up');
 
 const getESLintOptions = require('../utils/getESLintOptions');
@@ -98,9 +100,19 @@ const getComputedFixValue = ({ fix, quiet, fixDryRun }) => {
   return undefined;
 };
 
-const useFlatConfig = async () =>
-  !!(await findUp('eslint.config.js', { cwd: process.cwd() }));
-
+const useFlatConfig = async () => {
+  if (semver.satisfies(version, '>=8')) {
+    switch (process.env.ESLINT_USE_FLAT_CONFIG) {
+      case 'true':
+        return true;
+      case 'false':
+        return false;
+      default:
+        return !!(await findUp('eslint.config.js', { cwd: process.cwd() }));
+    }
+  }
+  return false;
+};
 const getESLintConstructor = async () => {
   if (await useFlatConfig()) {
     // Use a dynamic require here rather than a global require because this