- introduce new API allowing to write changelog directly to result file

- introduce test case for writing a file

__tests__/main.test.ts

@@ -1,6 +1,7 @@
 import * as path from 'path'
 import * as process from 'process'
 import * as cp from 'child_process'
+import * as fs from 'fs'
 
 test('missing values should result in failure', () => {
   process.env['GITHUB_WORKSPACE'] = '.'
@@ -17,7 +18,7 @@ test('missing values should result in failure', () => {
   }
 })
 
-test('missing values should result in failure', () => {
+test('complete input should succeed', () => {
   process.env['GITHUB_WORKSPACE'] = '.'
   process.env['INPUT_CONFIGURATION'] = 'configuration.json'
   process.env['INPUT_OWNER'] = 'mikepenz'
@@ -31,4 +32,29 @@ test('missing values should result in failure', () => {
   }
   const result = cp.execSync(`node ${ip}`, options).toString()
   // should succeed
+  expect(result).toBeDefined()
+})
+
+test('should write result to file', () => {
+  process.env['GITHUB_WORKSPACE'] = '.'
+  process.env['INPUT_CONFIGURATION'] = 'configuration.json'
+  process.env['INPUT_OWNER'] = 'mikepenz'
+  process.env['INPUT_REPO'] = 'release-changelog-builder-action'
+  process.env['INPUT_FROMTAG'] = 'v0.3.0'
+  process.env['INPUT_TOTAG'] = 'v0.5.0'
+  process.env['INPUT_OUTPUTFILE'] = 'test.md'
+
+  const ip = path.join(__dirname, '..', 'lib', 'main.js')
+  const options: cp.ExecSyncOptions = {
+    env: process.env
+  }
+  const result = cp.execSync(`node ${ip}`, options).toString()
+  // should succeed
+  expect(result).toBeDefined()
+
+  const readOutput = fs.readFileSync("test.md")
+
+  fs.unlinkSync("test.md")
+
+  expect(readOutput.toString()).not.toBe('')
 })