bug: asdf plugin test does not cleanup the test plugin

Describe the Bug

When I am actively developing my plugin and running asdf plugin test nim ., I have to manually run asdf plugin remove asdf-test-nim before each run.

I could see it being sometimes useful to keep the installed test plugin around if a plugin developer needs to inspect the installation, but that should be opt-in rather than opt-out. The default behavior should be to uninstall the plugin once the test is done, whether it succeeds or fails.

Steps to Reproduce

# From some plugin directory
asdf plugin test nim .
asdf plugin test nim .  # Fails because `asdf-test-nim` is already installed

Expected Behaviour

# From some plugin directory
asdf plugin test nim .
asdf plugin test nim .  # Works beause `asdf-test-nim` was uninstalled

Actual Behaviour

FAILED: nim was not properly installed reason: Plugin named asdf-test-nim already added

Environment

macos and linux

asdf plugins affected (if relevant)

asdf-nim