Bug fix: decode bytes object to string for subprocess.check_output output.

When running test "./tools/run_tests/run_tests.py -l c++ -c dbg" in python3
environment, the following exception is observed.

./tools/run_tests/run_tests.py -l c++ -c dbg
PASSED: make [time=114.9sec, retries=0:0]

Omitting EPOLLEXCLUSIVE tests

2020-12-01 10:45:40,806 detected port server running version 21
2020-12-01 10:45:40,866 my port server is version 21
Traceback (most recent call last):
  File "./tools/run_tests/run_tests.py", line 1919, in <module>
    build_only=args.build_only)
  File "./tools/run_tests/run_tests.py", line 1800, in _build_and_run
    one_run = set(spec for language in languages
  File "./tools/run_tests/run_tests.py", line 1801, in <genexpr>
    for spec in language.test_specs()
  File "./tools/run_tests/run_tests.py", line 383, in test_specs
    for line in tests.split('\n'):
TypeError: a bytes-like object is required, not 'str'

Signed-off-by: dapeng-mi <clark_mdp@163.com>
diff --git a/tools/run_tests/run_tests.py b/tools/run_tests/run_tests.py
index ac462df..9a8505d 100755
--- a/tools/run_tests/run_tests.py
+++ b/tools/run_tests/run_tests.py
@@ -355,7 +355,7 @@
                             tests = subprocess.check_output(
                                 [binary, '--benchmark_list_tests'],
                                 stderr=fnull)
-                        for line in tests.split('\n'):
+                        for line in tests.decode().split('\n'):
                             test = line.strip()
                             if not test: continue
                             cmdline = [binary,
@@ -380,7 +380,7 @@
                             tests = subprocess.check_output(
                                 [binary, '--gtest_list_tests'], stderr=fnull)
                         base = None
-                        for line in tests.split('\n'):
+                        for line in tests.decode().split('\n'):
                             i = line.find('#')
                             if i >= 0: line = line[:i]
                             if not line: continue