Prevent mix test from overriding :failures_manifest_path option

[sabiwara]

The issue is introduced in 99be673

The code is a bit complex because require_test_helper which might call ExUnit.configure is wrapped in the try/catch, which prevents it from assigning it to opts once in for all and pass it around.

https://github.com/elixir-lang/elixir/blob/main/lib/mix/lib/mix/tasks/test.ex#L663-L670

These two both now work:

# test/test_helper.exs
ExUnit.start(failures_manifest_path: "failed.txt")

# test/test_helper.exs
ExUnit.start(failures_manifest_path: "failed.txt")
raise "test the try/catch"

[josevalim]

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