x/mdview: bring test coverage from 7.0% to 80%

[skhal]

% check-go-test -cover 80 `find ./x/mdview f -name \*.go`
find: f: No such file or directory
=== COVERAGE: github.com/skhal/lab/x/mdview/internal/mdview
    coverage: 7.0% of statements
    threshold: 80.0%
--- FAIL