[7/7] docs(test): add FIXMEs detailing problems with `git test run` worktree management

[arxanas]

Stack:

• https://github.com/arxanas/git-branchless/pull/1353
• https://github.com/arxanas/git-branchless/pull/1354
• https://github.com/arxanas/git-branchless/pull/1355
• https://github.com/arxanas/git-branchless/pull/1356
• https://github.com/arxanas/git-branchless/pull/1202
• https://github.com/arxanas/git-branchless/pull/1357
• https://github.com/arxanas/git-branchless/pull/1358

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docs(test): add FIXMEs detailing problems with git test run worktree management