[core,utils] Support unpublicised `--no-check-extensions` #903

Job	Run time
Linter	17s
Select tests from inputs	1s
Run tests (ubuntu-20.04, 2.6, cpython, core, sh)	1m 19s
Run tests (ubuntu-20.04, 2.7, cpython, core, sh)	1m 7s
Run tests (ubuntu-20.04, 3.2, cpython, core, sh)	1m 23s
Run tests (ubuntu-20.04, 3.3, cpython, core, sh)	1m 28s
Run tests (ubuntu-20.04, 3.4, cpython, core, sh)	1m 20s
Run tests (ubuntu-20.04, 3.5, cpython, core, sh)	1m 4s
Run tests (ubuntu-20.04, 3.6, cpython, core, sh)	1m 14s
Run tests (ubuntu-20.04, 3.7, cpython, core, sh)	1m 2s
Run tests (ubuntu-20.04, 3.8, cpython, core, sh)	1m 3s
Run tests (ubuntu-20.04, 3.9, cpython, core, sh)	1m 10s
Run tests (ubuntu-20.04, 3.10, cpython, core, sh)	1m 15s
Run tests (ubuntu-20.04, 3.11, cpython, core, sh)	1m 0s
Run tests (ubuntu-20.04, 3.12, cpython, core, sh)	1m 7s
Run tests (ubuntu-20.04, pypy-2.7, cpython, core, sh)	2m 33s
Run tests (ubuntu-20.04, pypy-3.6, cpython, core, sh)	2m 33s
Run tests (ubuntu-20.04, pypy-3.7, cpython, core, sh)	2m 56s
Run tests (windows-2019, 3.4, cpython, core, bat)	1m 57s
Run tests (windows-2019, 3.4, cpython, nodownload, bat)	1m 16s
Run tests (ubuntu-20.04, 2.7, jython, core, sh)	3m 18s
Run tests (ubuntu-20.04, 2.7, jython, nodownload, sh)	21s
	30m 44s

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