Simplify tests by replacing output capture with asserts where possible

The C++ part of the test code is modified to achieve this. As a result,
this kind of test:

```python
with capture:
    kw_func1(5, y=10)
assert capture == "kw_func(x=5, y=10)"
```

can be replaced with a simple:

`assert kw_func1(5, y=10) == "x=5, y=10"`

tests/test_kwargs_and_defaults.py

@@ -5,77 +5,51 @@ from pybind11_tests import (kw_func0, kw_func1, kw_func2, kw_func3, kw_func4, ca
 
 
 def test_function_signatures(doc):
-    assert doc(kw_func0) == "kw_func0(arg0: int, arg1: int) -> None"
-    assert doc(kw_func1) == "kw_func1(x: int, y: int) -> None"
-    assert doc(kw_func2) == "kw_func2(x: int=100, y: int=200) -> None"
+    assert doc(kw_func0) == "kw_func0(arg0: int, arg1: int) -> str"
+    assert doc(kw_func1) == "kw_func1(x: int, y: int) -> str"
+    assert doc(kw_func2) == "kw_func2(x: int=100, y: int=200) -> str"
     assert doc(kw_func3) == "kw_func3(data: str='Hello world!') -> None"
-    assert doc(kw_func4) == "kw_func4(myList: List[int]=[13, 17]) -> None"
-    assert doc(kw_func_udl) == "kw_func_udl(x: int, y: int=300) -> None"
-    assert doc(kw_func_udl_z) == "kw_func_udl_z(x: int, y: int=0) -> None"
+    assert doc(kw_func4) == "kw_func4(myList: List[int]=[13, 17]) -> str"
+    assert doc(kw_func_udl) == "kw_func_udl(x: int, y: int=300) -> str"
+    assert doc(kw_func_udl_z) == "kw_func_udl_z(x: int, y: int=0) -> str"
     assert doc(args_function) == "args_function(*args) -> None"
     assert doc(args_kwargs_function) == "args_kwargs_function(*args, **kwargs) -> None"
     assert doc(KWClass.foo0) == "foo0(self: m.KWClass, arg0: int, arg1: float) -> None"
     assert doc(KWClass.foo1) == "foo1(self: m.KWClass, x: int, y: float) -> None"
 
 
-def test_named_arguments(capture, msg):
-    with capture:
-        kw_func1(5, 10)
-    assert capture == "kw_func(x=5, y=10)"
-    with capture:
-        kw_func1(5, y=10)
-    assert capture == "kw_func(x=5, y=10)"
-    with capture:
-        kw_func1(y=10, x=5)
-    assert capture == "kw_func(x=5, y=10)"
+def test_named_arguments(msg):
+    assert kw_func0(5, 10) == "x=5, y=10"
 
-    with capture:
-        kw_func2()
-    assert capture == "kw_func(x=100, y=200)"
-    with capture:
-        kw_func2(5)
-    assert capture == "kw_func(x=5, y=200)"
-    with capture:
-        kw_func2(x=5)
-    assert capture == "kw_func(x=5, y=200)"
-    with capture:
-        kw_func2(y=10)
-    assert capture == "kw_func(x=100, y=10)"
-    with capture:
-        kw_func2(5, 10)
-    assert capture == "kw_func(x=5, y=10)"
-    with capture:
-        kw_func2(x=5, y=10)
-    assert capture == "kw_func(x=5, y=10)"
+    assert kw_func1(5, 10) == "x=5, y=10"
+    assert kw_func1(5, y=10) == "x=5, y=10"
+    assert kw_func1(y=10, x=5) == "x=5, y=10"
+
+    assert kw_func2() == "x=100, y=200"
+    assert kw_func2(5) == "x=5, y=200"
+    assert kw_func2(x=5) == "x=5, y=200"
+    assert kw_func2(y=10) == "x=100, y=10"
+    assert kw_func2(5, 10) == "x=5, y=10"
+    assert kw_func2(x=5, y=10) == "x=5, y=10"
 
     with pytest.raises(TypeError) as excinfo:
         # noinspection PyArgumentList
         kw_func2(x=5, y=10, z=12)
     assert msg(excinfo.value) == """
         Incompatible function arguments. The following argument types are supported:
-            1. (x: int=100, y: int=200) -> None
+            1. (x: int=100, y: int=200) -> str
             Invoked with:
     """
 
-    with capture:
-        kw_func4()
-    assert capture == "kw_func4: 13 17"
-    with capture:
-        kw_func4(myList=[1, 2, 3])
-    assert capture == "kw_func4: 1 2 3"
+    assert kw_func4() == "{13 17}"
+    assert kw_func4(myList=[1, 2, 3]) == "{1 2 3}"
 
-    with capture:
-        kw_func_udl(x=5, y=10)
-    assert capture == "kw_func(x=5, y=10)"
-    with capture:
-        kw_func_udl_z(x=5)
-    assert capture == "kw_func(x=5, y=0)"
+    assert kw_func_udl(x=5, y=10) == "x=5, y=10"
+    assert kw_func_udl_z(x=5) == "x=5, y=0"
 
 
 def test_arg_and_kwargs(capture):
-    with capture:
-        call_kw_func(kw_func2)
-    assert capture == "kw_func(x=1234, y=5678)"
+    assert call_kw_func(kw_func2) == "x=1234, y=5678"
     with capture:
         args_function('arg1_value', 'arg2_value', 3)
     assert capture.unordered == """