Add interface to code coverage and combine tests

[menon-karthik]

Problem

The current code coverage does not include the C++ interface and its tests. We should fix this.

We might also want to streamline the three interface tests into one test. Pro: It is cleaner. Con: The current tests serve as examples on how to use the interface for different situations. Combining the tests will make them less useful as examples.

Solution

According to @mrp089, the including the interface tests in the code coverage might just require adding the code from the main CMakeLists to the CMakeLists inside tests/test_interface/.

Additional context

No response

Code of Conduct

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[mrp089]

For reference: We include coverage for svFSI and svFSILS in separate CMakeLists.txt as well, controlled by the main one.