Fix samba tests

[phil-davis]

Description

• enable the samba "unit" tests again
• mkdir /var/cache/samba - this is needed in Ubuntu 18.04 (current drone) as reported at https://bugs.launchpad.net/ubuntu/+source/gvfs/+bug/1758653 Actually the tests can pass without it, but the unit test output is full of "Permission denied" errors. See comment [WIP] Enable samba tests without mkdir #36425 (comment)
• enhance the test error messages in apps/files_external/tests/Storage/SmbTest.php - this was useful to try and understand exactly where things failed
• add 'domain'=>'WORKGROUP' to the smb config files. This is needed now, it seems that smbclient or samba-something may have provided this by default in the past, but it needs to be specified when using Ubuntu 18.04 with samba... - thanks to @jvillafanez for finding this.

Related Issue

• Fixes CI is failing #36322

Motivation and Context

Make CI great again.

How Has This Been Tested?

CI

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[micbar]

LGTM

[codecov]

Codecov Report

Merging #36423 into master will increase coverage by 0.3%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #36423     +/-   ##
===========================================
+ Coverage     64.87%   65.18%   +0.3%     
  Complexity    19792    19792             
===========================================
  Files          1271     1271             
  Lines         74723    74723             
  Branches       1309     1309             
===========================================
+ Hits          48479    48705    +226     
+ Misses        25858    25632    -226     
  Partials        386      386

Flag	Coverage Δ	Complexity Δ
#javascript	54% <ø> (ø)	0 <ø> (ø)	⬇️
#phpunit	66.41% <ø> (+0.33%)	19792 <ø> (ø)	⬇️

Impacted Files	Coverage Δ	Complexity Δ
apps/files_external/lib/Lib/Storage/SMB.php	52.19% <0%> (+52.19%)	0% <0%> (ø)	⬇️

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