Add test coverage of C files

[pmav99]

Disclaimer: I've only tested with gcc.

Using the info from these links I managed to get test coverage for the C codebase:

• ​https://stackoverflow.com/questions/16682606/how-to-resovle-gcov-init-undefined-reference-issue-when-link
• ​https://stackoverflow.com/questions/39086957/test-coverage-from-swig-compiled-c-python-code

1. install ​gcov
2. Compile with these options:

   export CFLAGS='-O0 -fPIC -march=native -std=gnu99 -fprofile-arcs -ftest-coverage -g'
   export LDFLAGS='-s -lgcov --coverage'
   

3. Run tests
4. Generate test coverage reports (there may be warning/errors):

   mkdir -p coverage
   gcovr -r . --filter="" --html --html-details -o coverage/coverage.html
   

5. Inspect coverage:

   firefox coverage/coverage.html
   

If possible, this should be added to fatra and once #3771 is resolved, on travis too.

I guess that it would also make sense to add a configure option like e.g. ​libspatialite:

#-----------------------------------------------------------------------
#    --enable-gcov
#
# Check whether --enable-gcov was given.
if test "${enable_gcov+set}" = set; then :
  enableval=$enable_gcov;
fi

if test "x$enable_gcov" = "xyes"; then
        CFLAGS=$CFLAGS" -fprofile-arcs -ftest-coverage -g"
    GCOV_FLAGS="-lgcov"
fi
#-----------------------------------------------------------------------

[pmav99]

For the record, GDAL is already using gcov/lcov, too, so it might make sense to see how they have integrated them: ​https://gdalautotest-coverage-results.github.io/coverage_html/index.html

[rouault]

GDAL is already using gcov/lcov, too, so it might make sense to see how they have integrated them

This is quite messy to be honest. Partly because I didn't try to make this better :-) So basically, I've a job that forks the GDAL master branch and re-applies the following changes on top of it : ​https://github.com/rouault/gdal_coverage/commit/3b63c70890a0897dcbe1cbd49f36b76e0a5e704a But the gist is build with lcov, run the tests, run lcov, filter some stuff in its output to have a clean report, run genhtml and then push this to a github repo. The interesting part that could be more or less reused by GRASS is in ​https://github.com/rouault/gdal_coverage/commit/3b63c70890a0897dcbe1cbd49f36b76e0a5e704a#diff-46c965c7db8bedae7518f42e783c44dd