Rename test folder to test (for evoke), fix warnings in pedantic

[dascandy]

• remove unused parameter names
• mark unused variables as maybe_unused
• resolve collision with catch's STATIC_REQUIRE
• make assignment operators return a value
• remove the "test.cpp" file which is lacking includes and excluded from build

and rename the test folder to "test", so that evoke picks it up and runs it (rather than just building it).

Checked with @TartanLlama beforehand for OK on the changes in principle.