return non-zero exit code if no tests are collected

Fix #812
Fix #500

testing/test_helpconfig.py

@@ -1,3 +1,4 @@
+from _pytest.main import EXIT_NOTESTSCOLLECTED
 import pytest
 
 def test_version(testdir, pytestconfig):
@@ -43,7 +44,7 @@ def test_hookvalidation_optional(testdir):
             pass
     """)
     result = testdir.runpytest()
-    assert result.ret == 0
+    assert result.ret == EXIT_NOTESTSCOLLECTED
 
 def test_traceconfig(testdir):
     result = testdir.runpytest("--traceconfig")
@@ -54,14 +55,14 @@ def test_traceconfig(testdir):
 
 def test_debug(testdir, monkeypatch):
     result = testdir.runpytest_subprocess("--debug")
-    assert result.ret == 0
+    assert result.ret == EXIT_NOTESTSCOLLECTED
     p = testdir.tmpdir.join("pytestdebug.log")
     assert "pytest_sessionstart" in p.read()
 
 def test_PYTEST_DEBUG(testdir, monkeypatch):
     monkeypatch.setenv("PYTEST_DEBUG", "1")
     result = testdir.runpytest_subprocess()
-    assert result.ret == 0
+    assert result.ret == EXIT_NOTESTSCOLLECTED
     result.stderr.fnmatch_lines([
         "*pytest_plugin_registered*",
         "*manager*PluginManager*"