Test runner: Capture test stderr

src/tests/test_utils/runner/README.md

@@ -61,10 +61,10 @@ During normal mode:
 During bot mode:
 
  * Tests are run in multiple processes depending on the system processor count.
- * A server process records the child processes' stdout.
+ * A server process records the child processes' stdout and stderr.
  * The server terminates a child process if there's no progress after a timeout.
  * The server sorts work into batches according to the back-end configuration.
- * This prevents driver errors from using mulitple back-ends in the same process.
+ * This prevents driver errors from using multiple back-ends in the same process.
  * Batches are striped to help split up slow groups of tests.
  * The server passes test batches to child processes via a `gtest_filter` file.
  * Bot mode does not work on Android or Fuchsia.

src/tests/test_utils/runner/TestSuite.cpp

@@ -1433,7 +1433,7 @@ bool TestSuite::launchChildTestProcess(uint32_t batchId,
     }
 
     // Launch child process and wait for completion.
-    processInfo.process = LaunchProcess(args, ProcessOutputCapture::StdoutAndStderrSeparately);
+    processInfo.process = LaunchProcess(args, ProcessOutputCapture::StdoutAndStderrInterleaved);
 
     if (!processInfo.process->started())
     {