update /docs/testing.rst based on new tutorial tests

[isdanni]

Update the /docs/testing.rst

Update the/docs/testing.rst. Remove the incompatible code snippet like this one since the flaskr application has been rewritten and does not contain a flaskr object.

a. Sections removed from /docs/testing.rst:

1. The Testing Skeleton;
2. The First Test;
3. Logging In and Out;
4. Test Adding Messages;

b. Replaced with:

1. The Testing Setup and Fixtures;
2. Database;
3. Authentication;
4. Blog;

c. Added Section:

• Factory; (The previous version referred to 0.12, which does not contain an app factory, could cause confusions)

Most description is directly taken from /docs/tutorial/tests.rst since @davidism mentioned here that the tutorial is update to date;

Relevant issues: #2831, #3545, and #2833 .
Possible overlaps: #3349 , #3311 .

NOTE: updating the testing docs seems like a long-term job and would take some effort to entirely make the testing docs compatible with the current flask version; I will continue study and update more outdated contents; Meanwhile, any suggestions and help are welcome! :)

[davidism]

Thanks for working on this. I ended up doing a full rewrite of these docs in #4448.