lint: convert format strings linter test to python

[Eunoia1729]

Refs #24783

Attempted to keep the style and flow of implementation as it is.

Additional Notes(Optional):

1. There is scope of improvement on how the related files are fetched. In this git grep with subprocess is still used as I found it to be the simplest. Any pointers on this are appreciated.
2. Removed sort operation on the matching files as I couldn't think of any strong arguments to have it. Any pointers on this are appreciated.
3. Not important, but one small detail is that the previous implementation was storing matched files for all the function_names iterated so far. Fixed that in this PR.

[laanwj]

There is scope of improvement on how the related files are fetched. In this git grep with subprocess is still used as I found it to be the simplest.

I think calling out to git grep (and git subcommands in general) is fine. The alternative would be to add a dependency on a git python module. Also, git is a single implementation on every platform (besides version differences). I've updated the guidelines in #24783 for this.

Removed sort operation on the matching files as I couldn't think of any strong arguments to have it. Any pointers on this are appreciated.

I don't know the original motivation, the only thing I can think of is that printing errors in with the files in a deterministic sorted order is somewhat-user-friendly. But I don't think it's a big deal.

[vincenzopalazzo]

utACK 879c836

[vincenzopalazzo]

Suggested change

	#!/usr/bin/env python3
	#
	# Copyright (c) 2018-2020 The Bitcoin Core developers
	# Distributed under the MIT software license, see the accompanying
	# file COPYING or http://www.opensource.org/licenses/mit-license.php.
	#
	"""
	Lint format strings: This program checks that the number of arguments passed
	to a variadic format string function matches the number of format specifiers
	in the format string.
	"""
	#!/usr/bin/env python3
	"""
	Lint format strings: This program checks that the number of arguments passed
	to a variadic format string function matches the number of format specifiers
	in the format string.
	Copyright (c) 2018-2020 The Bitcoin Core developers
	Distributed under the MIT software license, see the accompanying
	file COPYING or http://www.opensource.org/licenses/mit-license.php.
	"""

[Eunoia1729]

Previously ported python scripts have the copyright comment in the way, existing version of this PR has it. Changing it to the suggested way might make it appear inconsistent with other lint files. Do you still suggest changing ?

[maflcko]

From reading the code it looks like all output and stderr are muted. If yes, why? If no, please provide example stdout/stderr of the test before and after this change.

[Eunoia1729]

@MarcoFalke Thanks for highlighting that! That led me to update this PR so output matches with the older version.
Before change:

After change:

The reason why before version has 3 lines printed and not 2 is due to the issue highlighed in point (3) in the additional notes of PR header section. IMO the new version is the intended behavior.

[laanwj]

Code review ACK 267684e

[maflcko]

wouldn't it be better to use sys.executable?