revert 0e534c8, rewriting plugin tests to use actual pugins (#11713)

Revert https://mattermost.atlassian.net/browse/MM-14559. The tests were refactored, but all the hooks were be run as:

    _, ret := hooks.MessageWillBePosted(nil, nil)

and the plugins were panicking, but then nothing was actually failing because this hook doesn't support returning a "proper" error. Good news is that the old tests are still passing, so nothing has regressed in the toolkit, but thinking we should revisit this holistically instead of trying to fix in place.

Makefile

@@ -76,7 +76,7 @@ TESTFLAGS ?= -short
 TESTFLAGSEE ?= -short
 
 # Packages lists
-TE_PACKAGES=$(shell go list ./...|grep -v plugin_tests)
+TE_PACKAGES=$(shell go list ./...)
 
 # Plugins Packages
 PLUGIN_PACKAGES=mattermost-plugin-zoom-v1.0.7