You can build a test coverage report using the --enable-coverage
argument to ./configure, then running make coverage-report to
build an HTML coverage report in the coverage/ directory.
Note also that, because calculating the coverage requires compiler
support, you will need to clean any object files from a non-coverage
build by running make clean before make coverage-report.
For this to work, you will need to have lcov, gcov and genhtml
installed. On Ubuntu you can just run sudo apt-get install lcov to
get these.