Setup e2e testsuite

[mara-schulke]

Introduces

• Infrastructure for tests (VirtualFileSystem, macros, utils)
• A first fixture test for buffrs add

[uschi2000]

this looks very good, thank you! just minor comments on naming.

[uschi2000]

is this meant to be here?

[mara-schulke]

Yes it is! It's the asserted stdout log of the cli in this test. Is there any reason to delete it?

I think adjusting the test will be much easier when there is already the setup for present for asserting stdout logs.

[uschi2000]

I am not sure the logs are stable enough for equality assertion? Eg, timestamps will change, and even the order of entries might change (at least if there's any concurrent code).

I would consider stdout assertions relative low value; so if it's easy then we can include them, otherwise I don't think you loose much if you remove them.

[mara-schulke]

Asserting that the logs are empty does provide value; It ensures we don't add accidental log statements in the add command / helpful error messages are given in error cases

Also for concurrent operations eg cargo does not assert the logs / uses a different setup:

https://github.com/rust-lang/cargo/blob/master/tests/testsuite/concurrent.rs