Set working directory in lazygit test command

We need to fetch our list of tests both outside of our test binary and within. We need
to get the list from within so that we can run the code that drives the test and runs
assertions. To get the list of tests we need to know where the root of the lazygit repo
is, given that the tests live in files under that root.

So far, we've used this GetLazyRootDirectory() function for that, but it assumes that
we're not in a test directory (it just looks for the first .git dir it can find). Because
we didn't want to properly fix this before, we've been setting the working directory of
the test command to the lazygit root, and using the --path CLI arg to override it when
the test itself ran. This was a terrible hack.

Now, we're passing the lazygit root directory as an env var to the integration test, so
that we can set the working directory to the actual path of the test repo; removing the
need to use the --path arg.

pkg/integration/README.md

@@ -24,6 +24,8 @@ Each test has two important steps: the setup step and the run step.
 
 In the setup step, we prepare a repo with shell commands, for example, creating a merge conflict that will need to be resolved upon opening lazygit. This is all done via the `shell` argument.
 
+When the test runs, lazygit will open in the same working directory that the shell ends up in (so if you want to start lazygit somewhere other than the default location, you can use `shell.Chdir()` at the end of the setup step to set that working directory.
+
 ### Run step
 
 The run step has two arguments passed in: