Make a test fail to show the bug

[jonc125]

Description

Cope nicely if modellers stick a cmeta:id on a non-source variable that gets elided when resolving connections.

Motivation and Context

Fixes #267.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage

[codecov]

Codecov Report

Merging #269 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #269   +/-   ##
========================================
  Coverage    97.43%   97.44%           
========================================
  Files            7        7           
  Lines         1327     1329    +2     
  Branches       288      289    +1     
========================================
+ Hits          1293     1295    +2     
  Misses          15       15           
  Partials        19       19           

Impacted Files	Coverage Δ
cellmlmanip/model.py	99.54% <100.00%> (+<0.01%)	⬆️
cellmlmanip/units.py	98.40% <100.00%> (ø)

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[MauriceHendrix]

Works a treat thanks!