Improve the unittest stubs. (#1927)

* Improvements to stdlib/2/unittest.pyi

* Adds a bunch of missing things.
* Fixes the signatures of assertRaises and failUnlessRaises.
* Corrects the name of a TestCase subclass from CallableTestCase
  to FunctionTestCase.
* Makes defaultTestLoader an instance of TestLoader, as it should
  be, rather than an alias. Based on https://docs.python.org/2/library/unittest.html and
  https://github.com/python/cpython/tree/2.7/Lib/unittest.

* Improvements to stdlib/3/unittest/__init__.pyi

* The constructor arguments to TextTestResult are all required.
* There is a module-level load_tests() function.
* removeResult() returns a bool. Based on https://docs.python.org/3/library/unittest.html and
  https://github.com/python/cpython/tree/master/Lib/unittest.

stdlib/3/unittest/__init__.pyi

@@ -279,8 +279,8 @@ class TestResult:
                        outcome: Optional[_SysExcInfoType]) -> None: ...
 
 class TextTestResult(TestResult):
-    def __init__(self, stream: TextIO = ..., descriptions: bool = ...,
-                 verbosity: int = ...) -> None: ...
+    def __init__(self, stream: TextIO, descriptions: bool,
+                 verbosity: int) -> None: ...
 _TextTestResult = TextTestResult
 
 defaultTestLoader = ...  # type: TestLoader
@@ -324,10 +324,12 @@ def main(module: str = ..., defaultTest: _DefaultTestType = ...,
          buffer: Optional[bool] = ...,
          warnings: Optional[str] = ...) -> TestProgram: ...
 
+def load_tests(loader: TestLoader, tests: TestSuite,
+               pattern: Optional[str]) -> TestSuite: ...
 
 def installHandler() -> None: ...
 def registerResult(result: TestResult) -> None: ...
-def removeResult(result: TestResult) -> None: ...
+def removeResult(result: TestResult) -> bool: ...
 @overload
 def removeHandler() -> None: ...
 @overload