feat(runner): Config option to prevent logging to the console twice

This might be the case when Karma.run() is called inside the same
process/terminal as the server is running in (e.g. development
servers).

Fixes karma-runner#2121, karma-runner#2799

docs/config/01-configuration-file.md

@@ -661,6 +661,16 @@ Additional reporters, such as `growl`, `junit`, `teamcity` or `coverage` can be
 Note: Just about all additional reporters in Karma (other than progress) require an additional library to be installed (via NPM).
 
 
+## runnerLogging
+**Type:** Boolean
+
+**Default:** `true`
+
+**Description:** Prevent `Karma.run()` from logging test results to `stdout`.
+
+When `Karma.run()` is executed in the same process/terminal as the server, this option allows the developer to prevent the runner from logging the output to the terminal. This helps to avoid duplicated test result output in development servers.
+
+
 ## formatError
 **Type:** Function
 

lib/config.js

@@ -296,6 +296,7 @@ class Config {
     this.restartOnFileChange = false
     this.usePolling = process.platform === 'linux'
     this.reporters = ['progress']
+    this.runnerLogging = true
     this.singleRun = false
     this.browsers = []
     this.captureTimeout = 60000

lib/runner.js

@@ -54,7 +54,14 @@ function run (config, done) {
     response.on('data', function (buffer) {
       const parsedResult = parseExitCode(buffer, exitCode, config.failOnEmptyTestSuite)
       exitCode = parsedResult.exitCode
-      process.stdout.write(parsedResult.buffer)
+
+      // When Karma.run() is issued in the same process as the Karma server,
+      // test results will be logged twice: by the reporter and directly by
+      // the runner (here). Give the developer an option to disable this
+      // logging to prevent duplicated output.
+      if (config.runnerLogging !== false) {
+        process.stdout.write(parsedResult.buffer)
+      }
     })
 
     response.on('end', () => done(exitCode))