Fix tests failing on WSL systems.

[rsashank]

What does this PR do, and why?

Addressed issues with file path separators \\ and GUI return codes specific to WSL environments.

Fixes the failing of these tests on WSL environments, these have been failing for a while now but I just got used to ignoring them, decided to look into them today and fix. 😓

Outstanding aspect(s)

• [ ]

External discussion & connections

• [x] Discussed in #zulip-terminal in WSL specific test fails
• [ ] Fully fixes #
• [ ] Partially fixes issue #
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [ ] Merge will enable work on #

How did you test this?

• [ ] Manually - Behavioral changes
• [ ] Manually - Visual changes
• [ ] Adapting existing automated tests
• [ ] Adding automated tests for new behavior (or missing tests)
• [ ] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [ ] It is a minimal coherent idea
• [ ] It has a commit summary following the documented style (title & body)
• [ ] It has a commit summary describing the motivation and reasoning for the change
• [ ] It individually passes linting and tests
• [ ] It contains test additions for any new behavior
• [ ] It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes