bump cov - update `ci.yml` - fix tests on local build

[t-bltg]

• improve coverage, simplify some tests;
• update ci.yml (use action@latest where possible, and julia-actions/cache@v1);
• julia: drop 1.0, min is now 1.6 LTS (JuliaInterpreter, Cthulhu, Debugger, Revise ✔: this will require bump to 1.1.0), drop 1.0 compat code;
• remove unnecessary joinpath in utils.jl (abspath already joins paths);
• fix @test occursin(Sys.STDLIB, whereis(m)[1]) on a local julia build where the source files are available locally on the fs.

[t-bltg]

@timholy, I guess you missed this PR, too bad for 1.1.0 (https://github.com/timholy/CodeTracking.jl/commit/426fc0e5af69ca410f3a1b77458db5fc5d68e864).

[timholy]

Sorry I missed this, and thanks for the very nice cleanup!