(Closes #1770) make eg20 compile

[arporter]

Adds a couple of extra files to our test copy of LFRic infrastructure and extends eg20 so that it compiles and links the example.

[arporter]

A small PR that just extends the LFRic test infrastructure and then uses it to get eg20 to compile. One for @hiker :-)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (533ef7b) to head (772d102).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #3603   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          402       402           
  Lines        56693     56693           
=========================================
  Hits         56693     56693           

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[hiker]

Simple change. The code compiles and cleans as expected. I've triggered the IT, and will merge when this is green (CT linkcheck failures are caused by known issues with zeonodo atm).