Cleanup irb history file in tests

[apainintheneck]

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This was caused by the irb spec which I forgot to update while working on this command a few days ago. Note that $HOME is set during the test setup so will always be correct.

Before

/u/l/Homebrew (master|✔) $ git status
On branch master
Your branch is up to date with 'origin/master'.

nothing to commit, working tree clean
/u/l/Homebrew (master|✔) $ brew tests --only=dev-cmd/irb
Randomized with seed 26142
1 process for 1 spec, ~ 1 spec per process
..
Took 5 seconds
/u/l/Homebrew (master|✔) $ git status
On branch master
Your branch is up to date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        Library/Homebrew/test/.brew_irb_history

nothing added to commit but untracked files present (use "git add" to track)

After

/u/l/Homebrew (master|✔) $ git checkout fix-irb-test
Switched to branch 'fix-irb-test'
/u/l/Homebrew (fix-irb-test|✔) $ brew tests --only=dev-cmd/irb
Randomized with seed 26186
1 process for 1 spec, ~ 1 spec per process
..
Took 5 seconds
/u/l/Homebrew (fix-irb-test|✔) $ git status
On branch fix-irb-test
nothing to commit, working tree clean

[BrewTestBot]

Review period will end on 2023-03-13 at 01:16:57 UTC.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Thanks again @apainintheneck!