New Praefect doctor diagnostic

What does this Merge Request do and why?

This MR adds a gdk doctor diagnostic to check the length of the directory where praefect looks to create a UNIX socket as it's known to cause issues, see #1246 (closed). The exact number where things go awry is unknown but any improvement here is a good thing.

e.g. I hardcoded MAX_PRAEFECT_INTERNAL_SOCKET_DIR_LENGTH to 1 to show what it looks like:

$ gdk doctor
.............
⚠️  WARNING: GDK may need attention:

================================================================================
Please note these warning only exist for debugging purposes and can
help you when you encounter issues with GDK.
If your GDK is working fine, you can safely ignore them. Thanks!
================================================================================

Praefect
--------------------------------------------------------------------------------
Praefect will attempt to create UNIX sockets in:

  /Users/ash/src/gitlab/gitlab-development-kit/tmp/praefect

UNIX socket creation issues have been observed when the character length of the
directory above is < 1, yours is 57.

If you're experiencing issues, please try and reduce the directory depth to
be under 1 characters.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #809 (closed)