TestShell: Simplify default setting of num_nodes and return self from setup()

Summary:

This allows user to chain setup() to the initializer. test-shell.md code
examples have been updated to reflect this.

The typo related to backquotes used in example code was already fixed in D8247
One of the two code examples in test-shell.md was intentionally left "unchained", because it is useful to illustrate both ways of doing and because our version is much longer due to the necessity of specifying the config.ini path.

This is a backport of Core PR17378

Depends on D8247

Test Plan: ninja && ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8248