feat: set env var TS_JEST to allow detecting of ts-jest (#2717)

Closes #2716

Author: B4nan

src/ts-jest-transformer.spec.ts

@@ -352,6 +352,10 @@ Array [
       `)
     })
 
+    test('should allow detection of ts-jest', () => {
+      expect(process.env.TS_JEST).toBe('1')
+    })
+
     test.each(['foo.ts', 'foo.tsx'])('should process ts/tsx file', (filePath) => {
       const fileContent = 'const foo = 1'
       const output = 'var foo = 1'

src/ts-jest-transformer.ts

@@ -63,6 +63,7 @@ export class TsJestTransformer implements SyncTransformer {
     this.process = this.process.bind(this)
 
     this._logger.debug('created new transformer')
+    process.env.TS_JEST = '1'
   }
 
   private _configsFor(transformOptions: TransformOptionsTsJest): ConfigSet {