Fix warnings, add comments in fvectorconversion1d.cc test

This is a regression test, the threw up undefined variable warnings. unsure about the correct fix, I tried to understand the test. In addition to fixing the warnings, I left some comments too, in the hope future readers will have a little easier time understanding what it is supposed to do.

The warnings showed up in https://gitlab.dune-project.org/core/dune-common/-/jobs/139437#L665 (debian:10 clang-7-libcpp-17):

[ 60%] Building CXX object dune/common/test/CMakeFiles/fvectorconversion1d.dir/fvectorconversion1d.cc.o
/builds/core/dune-common/dune/common/test/fvectorconversion1d.cc:86:16: warning: unused variable 'mfv' [-Wunused-variable]
      MiddleFV mfv;
               ^
/builds/core/dune-common/dune/common/test/fvectorconversion1d.cc:107:16: warning: unused variable 'mfv' [-Wunused-variable]
      MiddleFV mfv;
               ^
2 warnings generated.

See also: !735 (merged), !562 (closed)