build: add test-coverage script

[ryzhak]

This PR adds a new test-coverage script to makefile which runs unit/doc tests and generates an html report locally in the target/llvm-cov/html folder.

This way it's easier to see parts of code which could be better covered with tests.

Notice that the displayed line coverage right now is 62.28% but that's not an accurate value because llvm-cov counts all unit tests in mod tests modules as uncovered code. Those unit tests should be manually excluded from the coverage report (which could be a part of another issue if collecting test coverage lies within the project vision):

#[cfg(test)]
#[cfg_attr(coverage_nightly, coverage(off))]
mod tests {
    // ...
}