mavsdk_tests: collect buffered up stdout output

Somehow only subprocess.stdout.readline() works at a time. In order not
to miss out on some of the stdout output, we need to collect it all at
the end. Also, we can stop using readline() for processes that have quit
already.

test/mavsdk_tests/mavsdk_test_runner.py

@@ -341,6 +341,8 @@ class Tester:
             is_success = False
 
         self.stop_runners()
+        # Collect what was left in output buffers.
+        self.collect_runner_output()
         self.stop_combined_log()
 
         result = {'success': is_success,

test/mavsdk_tests/process_helper.py

@@ -53,7 +53,7 @@ class Runner:
             cwd=self.cwd,
             env=self.env,
             stdout=subprocess.PIPE,
-            stderr=subprocess.STDOUT,
+            stderr=subprocess.PIPE,
             universal_newlines=True
         )
 
@@ -63,11 +63,12 @@ class Runner:
 
     def process_output(self) -> None:
         assert self.process.stdout is not None
-        while not self.stop_thread.is_set():
+        while True:
             line = self.process.stdout.readline()
-            if line == "\n":
-                continue
-            if not line:
+            if not line and \
+                    (self.stop_thread.is_set() or self.poll is not None):
+                break
+            if not line or line == "\n":
                 continue
             self.output_queue.put(line)
             self.log_fd.write(line)
@@ -99,8 +100,6 @@ class Runner:
         if not self.stop_thread:
             return 0
 
-        self.stop_thread.set()
-
         returncode = self.process.poll()
         if returncode is None:
 
@@ -123,8 +122,8 @@ class Runner:
             print("{} exited with {}".format(
                 self.cmd, self.process.returncode))
 
+        self.stop_thread.set()
         self.thread.join()
-
         self.log_fd.close()
 
         return self.process.returncode