make plugin test support testing without requiring it to be in the git tree

[airtonix]

Is your feature request related to a problem? Please describe

trying to iterate and it turns out i have to commit.every.single.change before i can test that the plugin works.

asdf plugin test <theplugin> .

this is kinda redonkulous:

Describe the proposed solution

a flag that just uses what ever is in the current directory instead of what ever is only in git tree.

Describe similar asdf features and why they are not sufficient

?

Describe other workarounds you've considered

going into the mountains and becoming a goat herder

[hyperupcall]

Duplicate of #1488

[hyperupcall]

I suppose not precisely a duplicate. This proposed adding a flag; linked issue proposes changing the feature