fix(discovery): test if file contains test

[Dynge]

The method to determine if the file is a test file did not take into account that it actually had tests for the file. It just checked that the file is from a project with a client.
There may be files that are not test files within the project.

[Dynge]

Oh I just saw that #37 also adresses this.

In my test it did not slow down my test discovery significantly for a project with over 700 tests.

That project used the mtp client, so I guess the vstest client may be slower.

I'll check if the file is updated to become a test file later if it starts to contain tests.

[Dynge]

Neotest does indeed rescan the file and adds it to the list. It is not immediate, I assumes theres some sort of worker periodically checking the files.

[Dynge]

@Nsidorenco any thoughts on this PR? :)

[Nsidorenco]

Thanks for the nudge @Dynge!

I'm a little worried that something like this would make the discovery process sluggish if a project contains many solutions but from testing out the branch it seems to work fine.

Let's merge this and potentially re-evaluate if issues arise