Fix C++ test Templates1

[tautschnig]

Add the missing include.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.04% 🎉

Comparison is base (d84744f) 78.08% compared to head (7d641ea) 78.12%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7851      +/-   ##
===========================================
+ Coverage    78.08%   78.12%   +0.04%     
===========================================
  Files         1699     1699              
  Lines       195105   195105              
===========================================
+ Hits        152338   152418      +80     
+ Misses       42767    42687      -80     

see 67 files with indirect coverage changes

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