Add checks to ensure test and section names do not clobber already defined units

[AutonomicPerfectionist]

Is your feature request related to a problem? Please describe.
When adding new tests, it's common to copy the definition of a previous test and modify the contents. However, sometimes the test writer forgets to change the name of the new section or test, and it overwrites the old test definition.

Describe the solution you'd like
Inside ct_add_section, we can do a quick check to see if the given name is already a section in the parent unit.

Alternatively, we could do a cpp_type_of() to check if the function with that name is already defined, this would additionally catch any errors with the weird scoping rules.

[AutonomicPerfectionist]

After #91 this is less of an issue, but we can still do verification in ct_add_section by checking the sibling sections' names