Allow module tests to be run from VS Code

[matt2e]

We are close to having module unit tests work within VS Code. This will also make it easy to debug module code/tests.
The only thing missing is that ftltest.WithProjectFiles() relies on FTL_CONFIG envar.

Workarounds:

• Set the variable in hermit, open vscode with code .
• Set it up as an envar in VS Code's settings json
  Both of these are annoying because you have to make sure you don't commit the changes because the path includes your home directory.

Possible solution:
We switch logic to look for expected file names in the current folder, if missing go up a level ... repeat.
This is similar to how you can call just from anywhere within the directory structure
We could make FTL_CONFIG and ftltest.WithProjectFiles(...) do this. This would make passing in strings to the test function actually usable in more cases, and the ftl config value to be able to be commited to git

[matt2e]

from discussion with @mistermoe

[alecthomas]

The current logic when running ftl dev is here. We should probably replicate that.

• If a config is passed via either the CLI or envar use them.
• Otherwise use `internal.GitRoot() + "/ftl-project.toml" if it exists.

[alecthomas]

In fact we should probably use ConfigPaths() directly...

[matt2e]

good first issue?