Making a nox -s default session for all packages. (#4324)

* Making a `nox -s default` session for all packages.

* Using "default" `nox` session on AppVeyor.

This was 32-bit or 64-bit Python can be used, depending
on which is the active `python` / the active `nox.exe`.

Author: dhermes

nox.py

@@ -26,6 +26,33 @@
 )
 
 
+@nox.session
+def default(session):
+    """Default unit test session.
+
+    This is intended to be run **without** an interpreter set, so
+    that the current ``python`` (on the ``PATH``) or the version of
+    Python corresponding to the ``nox`` binary the ``PATH`` can
+    run the tests.
+    """
+    # Install all test dependencies, then install this package in-place.
+    session.install('mock', 'pytest', 'pytest-cov', *LOCAL_DEPS)
+    session.install('-e', '.')
+
+    # Run py.test against the unit tests.
+    session.run(
+        'py.test',
+        '--quiet',
+        '--cov=google.cloud.error_reporting',
+        '--cov=tests.unit',
+        '--cov-append',
+        '--cov-config=.coveragerc',
+        '--cov-report=',
+        '--cov-fail-under=97',
+        'tests/unit',
+    )
+
+
 @nox.session
 @nox.parametrize('py', ['2.7', '3.4', '3.5', '3.6'])
 def unit(session, py):
@@ -37,16 +64,7 @@ def unit(session, py):
     # Set the virtualenv dirname.
     session.virtualenv_dirname = 'unit-' + py
 
-    # Install all test dependencies, then install this package in-place.
-    session.install('mock', 'pytest', 'pytest-cov', *LOCAL_DEPS)
-    session.install('-e', '.')
-
-    # Run py.test against the unit tests.
-    session.run(
-        'py.test', '--quiet', '--cov=google.cloud.error_reporting',
-        '--cov=tests.unit', '--cov-append', '--cov-config=.coveragerc',
-        '--cov-report=', '--cov-fail-under=97', 'tests/unit',
-    )
+    default(session)
 
 
 @nox.session