Fix plugin /public handling for subpaths (#21886)

* Fix plugin /public handling for subpaths

Plugins support a `public` folder in the bundle automatically being accessible at `<site_url>/plugins/<plugin_id/public/`, but it seems support for a `site_url` with a subpath has been broken for some time.

I tried to figure out when this stopped working, but gave up after a while and just focussed on the requisite changes plus tests.

* simplify comment

* more testing coverage, and simpler diff

* try cleaning up plugins after tests

* skip TestServePluginPublicRequest to isolate build issue

* Revert "skip TestServePluginPublicRequest to isolate build issue"

This reverts commit 62d0e4e427c7cf7b7f9b09202ce10cd5f1a56bca.

* do th.TearDown last by using t.Cleanup vs. defer

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

tests/README.md

@@ -22,18 +22,19 @@ It is possible to manually test specific sections of any test, instead of using
 There are two test plugins: `testplugin.tar.gz` and `testplugin2.tar.gz`. These are use in some integration tests in the `api4` package. Any changes to the plugin bundles require updating the corresponding signatures.
 
 First, import the public and private development key:
-```
-$ gpg --import ./development-public-key.gpg
-$ gpg --import ./development-private-key.asc
+```sh
+gpg --import ./development-public-key.gpg
+gpg --import ./development-private-key.asc
 ```
 
 This has to be done only once.
 
 Then update the signatures:
+```sh
+gpg -u F3FACE45E0DE642C8BD6A8E64C7C6562C192CC1F --verbose --personal-digest-preferences SHA256 --detach-sign testplugin.tar.gz
+gpg -u F3FACE45E0DE642C8BD6A8E64C7C6562C192CC1F --verbose --personal-digest-preferences SHA256 --detach-sign --armor testplugin.tar.gz
+gpg -u F3FACE45E0DE642C8BD6A8E64C7C6562C192CC1F --verbose --personal-digest-preferences SHA256 --detach-sign testplugin2.tar.gz
+gpg -u F3FACE45E0DE642C8BD6A8E64C7C6562C192CC1F --verbose --personal-digest-preferences SHA256 --detach-sign --armor testplugin2.tar.gz
 ```
-$ gpg -u F3FACE45E0DE642C8BD6A8E64C7C6562C192CC1F --verbose --personal-digest-preferences SHA256 --detach-sign testplugin.tar.gz
-$ gpg -u F3FACE45E0DE642C8BD6A8E64C7C6562C192CC1F --verbose --personal-digest-preferences SHA256 --detach-sign --armor testplugin.tar.gz
-$ gpg -u F3FACE45E0DE642C8BD6A8E64C7C6562C192CC1F --verbose --personal-digest-preferences SHA256 --detach-sign testplugin2.tar.gz
-$ gpg -u F3FACE45E0DE642C8BD6A8E64C7C6562C192CC1F --verbose --personal-digest-preferences SHA256 --detach-sign --armor testplugin2.tar.gz
 
 Finally, include the updates bundles and signatures in your commit.

tests/testplugin.tar.gz.asc

@@ -1,14 +1,14 @@
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-ElLLqMat
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