Docs: hourglass model in test principle

[charlie0129]

Signed-off-by: Charlie Chiang charlie_c_0129@outlook.com

I think hourglass model represents a large number of unit + e2e test, but no integration test better.

Reference: https://testing.googleblog.com/2015/04/just-say-no-to-more-end-to-end-tests.html

Inverted pyramid/ice cream cone. The team relies primarily on end-to-end tests, using few integration tests and even fewer unit tests.
Hourglass. The team starts with a lot of unit tests, then uses end-to-end tests where integration tests should be used. The hourglass has many unit tests at the bottom and many end-to-end tests at the top, but few integration tests in the middle.