Tests coverage

[kachayev]

The simplest test suite should contain the directory with files and at least a single nested directory. Server runner should be refactored into a separate function, so the test might start a new server on a random port and send requests using Aleph HTTP client.

Would be nice to have coverage for the following scenarios:

• open directory
• open file
• last modified flow with 304
• different content types
• empty file
• non-existing file & directory
• attempt to fetch a file from the parent directory