Use random tmp names for index files in tests

[achirkin]

Replace all hard-coded index file names in tests with a randomly-generated unique paths in the system temporary folder and delete the files after use.

This is a slightly opinionated convenience PR to give the following benefits:

• Temporary index files do not clutter the project folder
• One can run multiple test instances in parallel without the danger of data corruption (e.g. for stress-testing)
• The tests do not fail if the current folder is read-only

[achirkin]

/merge