cram tests: use file tests if possible (#5557)

Some tests use directory tests (`name.t/run.t`) when a plain file test
(`run.t`) would be enough. This renames them to simplify the directory
structure.

Code used:

    fix() {
        dir=$(dirname $1)
        git mv $dir $dir.tmp
        git mv $dir.tmp/run.t $dir
    }
    find test -type f -name run.t | while read -r f ; do
        [ $(ls -1q $(dirname $f)|wc -l) -eq 1 ] && fix $f ;
    done

Signed-off-by: Etienne Millon <me@emillon.org>