tests/run-tests: Capture any output from a crashed uPy execution.

Instead of putting just 'CRASH' in the .py.out file, this patch makes it so
any output from uPy that led to the crash is stored in the .py.out file, as
well as the 'CRASH' message at the end.

tests/run-tests

@@ -54,6 +54,7 @@ def run_micropython(pyb, args, test_file, is_special=False):
         'micropython/meminfo.py', 'basics/bytes_compare3.py',
         'basics/builtin_help.py', 'thread/thread_exc2.py',
     )
+    had_crash = False
     if pyb is None:
         # run on PC
         if test_file.startswith(('cmdline/', 'feature_check/')) or test_file in special_tests:
@@ -128,8 +129,9 @@ def run_micropython(pyb, args, test_file, is_special=False):
             # run the actual test
             try:
                 output_mupy = subprocess.check_output(cmdlist, stderr=subprocess.STDOUT)
-            except subprocess.CalledProcessError:
-                output_mupy = b'CRASH'
+            except subprocess.CalledProcessError as er:
+                had_crash = True
+                output_mupy = er.output + b'CRASH'
 
             # clean up if we had an intermediate .mpy file
             if args.via_mpy:
@@ -142,13 +144,14 @@ def run_micropython(pyb, args, test_file, is_special=False):
         try:
             output_mupy = pyb.execfile(test_file)
         except pyboard.PyboardError:
+            had_crash = True
             output_mupy = b'CRASH'
 
     # canonical form for all ports/platforms is to use \n for end-of-line
     output_mupy = output_mupy.replace(b'\r\n', b'\n')
 
     # don't try to convert the output if we should skip this test
-    if output_mupy in (b'SKIP\n', b'CRASH'):
+    if had_crash or output_mupy in (b'SKIP\n', b'CRASH'):
         return output_mupy
 
     if is_special or test_file in special_tests: