tests: Remove hardcoded -ftest-coverage

Continuous doesn't include -lgcov at the moment.  I could easily
enough, but really it'd be better for developers to just do this
locally with e.g.:

$ jhbuild make CFLAGS='-ftest-coverage'

or so.
build: Don't explicitly dist INSTALL

That happens by default for tarballs.  When building from git, this
causes INSTALL to be a build target, but the autotools may not install
the file.  This fixes a build failure in Continuous.