debug: use human readable dates for filenames

The unix timestamps that were used make the debug data a little bit more
difficult to consume. By using human readable dates we can easily see
when the profile data was collected.

This commit also improves the test coverage. Two test cases are removed
and the assertions from those cases are moved to TestDebugCommand.

Now TestDebugCommand is able to validate the contents of all files. This
change reduces the test runtime of the command/debug package by almost
50%. It also makes much more strict assertions about the contents by
using gotest.tools/v3/fs.