Add additional unit tests for appender string

[GalaxyRider111]

Adding some more additional unit test for string appender for the issue #8789 (https://github.com/dlang/phobos/pull/8789).

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Testing this PR locally

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dub run digger -- build "master + phobos#10763"

[Herringway]

There seems to be more than just unit tests in this PR.

[pbackus]

• Why does this PR contain both unit tests and functional changes, when the title and description only mention unit tests?
• Why does this PR contain changes to both appender and std.container.array, when the title and description only mention appender?
• What is the purpose of these changes? A PR description should explain why changes are being made, not just say what the changes are.

[GalaxyRider111]

• Why does this PR contain both unit tests and functional changes, when the title and description only mention unit tests?
• Why does this PR contain changes to both appender and std.container.array, when the title and description only mention appender?
• What is the purpose of these changes? A PR description should explain why changes are being made, not just say what the changes are.

Ok so, I'm sorry everyone, I have made a mistake. Initially, I added more unit tests for this PR. Then, I wanted to contribue to another issue, one that involves adding support for move-only types, and I have been working on this, hence the changes from std/container/array.d. Unfortunately, I kept working on the same branch as the one from this PR, my mistake, sorry for confusion.

[LightBender]

This PR has had a broken CI for 6+ months, please reopen if you plan to continue working on this.