Flush the output stream before reading the command line output.

This doesn't quite fully resolve the problem that not all
the process output will be guaranteed to have been read,
but it's less likely to lose the race if the the process
ends _really quickly_.

Author: shs96c

java/client/src/org/openqa/selenium/os/UnixProcess.java

@@ -153,6 +153,8 @@ public void waitFor(long timeout) throws InterruptedException {
       throw new InterruptedException(
           String.format("Process timed out after waiting for %d ms.", timeout));
     }
+
+    // Wait until syserr and sysout have been read
   }
 
   public boolean isRunning() {
@@ -178,6 +180,11 @@ public String getStdOut() {
       throw new IllegalStateException(
           "Cannot get output before executing command line: " + cl);
     }
+    try {
+      inputOut.flush();
+    } catch (IOException e) {
+      throw new RuntimeException(e);
+    }
     return inputOut.toString();
   }