TestShell: Return self from setup()

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

Author: jachiang

test/functional/test-shell.md

@@ -51,8 +51,7 @@ The following sections demonstrate how to initialize, run, and shut down a
 ## 3. Initializing a `TestShell` object
 
 ```
->>> test = TestShell()
->>> test.setup(num_nodes=2, setup_clean_chain=True)
+>>> test = TestShell().setup(num_nodes=2, setup_clean_chain=True)
 20XX-XX-XXTXX:XX:XX.XXXXXXX TestFramework (INFO): Initializing test directory /path/to/bitcoin_func_test_XXXXXXX
 ```
 The `TestShell` forwards all functional test parameters of the parent
@@ -66,8 +65,7 @@ temporary folder. If you need more bitcoind nodes than set by default (1),
 simply increase the `num_nodes` parameter during setup.
 
 ```
->>> test2 = TestShell()
->>> test2.setup()
+>>> test2 = TestShell().setup()
 TestShell is already running!
 ```
 

test/functional/test_framework/test_shell.py

@@ -42,6 +42,7 @@ def setup(self, **kwargs):
 
             super().setup()
             self.running = True
+            return self
 
         def shutdown(self):
             if not self.running: