docs: update HACKING.md to recurse submodules

[mr-cal]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

---

A minor HACKING.md update requested by @giusebar.

I recently added snapd-testing-tools as a submodule. This submodule is required for running spread tests, so now I've updated the hacking instructions.

[codecov-commenter]

Codecov Report

Merging #4006 (c638c18) into main (31a3990) will increase coverage by 0.61%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4006      +/-   ##
==========================================
+ Coverage   90.99%   91.61%   +0.61%     
==========================================
  Files          56       77      +21     
  Lines        4154     5153     +999     
==========================================
+ Hits         3780     4721     +941     
- Misses        374      432      +58     

Impacted Files	Coverage Δ
snapcraft/__init__.py	72.72% <0.00%> (ø)
snapcraft/plugins/v2/__init__.py	0.00% <0.00%> (ø)
snapcraft/commands/__init__.py	100.00% <0.00%> (ø)
snapcraft/parts/plugins/__init__.py	100.00% <0.00%> (ø)
snapcraft/errors.py	91.17% <0.00%> (ø)
snapcraft/ua_manager.py	96.38% <0.00%> (ø)
snapcraft/plugins/v1/__init__.py	0.00% <0.00%> (ø)
snapcraft/store/__init__.py	100.00% <0.00%> (ø)
snapcraft/repo/__init__.py	100.00% <0.00%> (ø)
snapcraft/repo/apt_sources_manager.py	94.73% <0.00%> (ø)
... and 11 more

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[mr-cal]

Failing ruby and flutter spread tests on core18 are unrelated. Merging.