Refactor known_failures.py to use a compact DSL instead of deeply nested conditions.

Part of #1367

docs/changes/1367.misc

+Refactor ``known_failures.py`` to use a compact DSL instead of deeply
+nested condition statements.

src/gevent/testing/monkey_test.py

@@ -13,15 +13,17 @@ monkey.patch_all()
 
 from .sysinfo import RUNNING_ON_APPVEYOR
 from .sysinfo import PY37
+from .sysinfo import PYPY3
 from .patched_tests_setup import disable_tests_in_source
 from . import support
 from . import resources
 from . import SkipTest
 from . import util
 
-if RUNNING_ON_APPVEYOR and PY37:
+if (RUNNING_ON_APPVEYOR and PY37) or PYPY3:
     # 3.7 added a stricter mode for thread cleanup.
     # It appears to be unstable on Windows (at least appveyor)
+    # and PyPy3
     # and test_socket.py constantly fails with an extra thread
     # on some random test. We disable it entirely.
     # XXX: Figure out how to make a *definition* in ./support.py actually

src/gevent/testing/testrunner.py

@@ -32,10 +32,11 @@ except (ImportError, OSError, IOError):
     pass
 
 TIMEOUT = 100 # seconds
-DEFAULT_NWORKERS = int(os.environ.get('NWORKERS') or max(cpu_count() - 1, 4))
-if DEFAULT_NWORKERS > 10:
+AVAIL_NWORKERS = cpu_count() - 1
+DEFAULT_NWORKERS = int(os.environ.get('NWORKERS') or max(AVAIL_NWORKERS, 4))
+if DEFAULT_NWORKERS > 15:
     DEFAULT_NWORKERS = 10
-SUGGESTED_NWORKERS = DEFAULT_NWORKERS
+
 
 if RUN_LEAKCHECKS:
     # Capturing the stats takes time, and we run each
@@ -149,7 +150,7 @@ class Runner(object):
     def __call__(self):
         util.log("Running tests in parallel with concurrency %s %s." % (
             self._worker_count,
-            util._colorize('number', '(concurrency available: %d)' % SUGGESTED_NWORKERS)
+            util._colorize('number', '(concurrency available: %d)' % AVAIL_NWORKERS)
         ),)
         # Setting global state, in theory we can be used multiple times.
         # This is fine as long as we are single threaded and call these