StDebuggerExtensionMechanismTest does not clear the debuggers it instanciates, causing errors because it could be still subscribed to extension (de)activation announcements when its context is nil

[adri09070]

StDebuggerExtensionMechanismTest
 ✗ #testUsesExtensions (17ms)
#isDead was sent to nil
UndefinedObject(Object)>>doesNotUnderstand: #isDead
StSindarinBytecodeDebuggerPresenter>>updatePresenter
StSindarinBytecodeDebuggerPresenter(SpPresenter)>>initializePrivateHooks
StSindarinBytecodeDebuggerPresenter(SpPresenter)>>initialize
StSindarinBytecodeDebuggerPresenter class(SpPresenter class)>>owner:on:
StDebugger(SpPresenter)>>instantiate:on:
StDebugger>>instantiateExtensionToolsPage:
[
		extensionToolsNotebook pages:
			{ (self instantiateExtensionToolsPage: extensionToggled) }
				asOrderedCollection.
		extensionToolsNotebook selectPageIndex: 1.
		stackAndCodeContainer := self stackAndCodeWithExtensionsLayout.
		self layout: self defaultLayout.
		^ self ] in StDebugger>>updateExtensionsFromAnnouncement:
OrderedCollection(Collection)>>ifEmpty:
StDebugger>>updateExtensionsFromAnnouncement:
WeakMessageSend>>value:
WeakMessageSend(MessageSend)>>cull:
WeakMessageSend(MessageSend)>>cull:cull:
[ action cull: anAnnouncement cull: announcer ] in WeakAnnouncementSubscription(AbstractAnnouncementSubscription)>>deliver:
FullBlockClosure(BlockClosure)>>on:do:
FullBlockClosure(BlockClosure)>>on:fork:
WeakAnnouncementSubscription(AbstractAnnouncementSubscription)>>deliver:
[ subscription deliver: anAnnouncement ] in SubscriptionRegistry>>deliver:to:startingAt:
FullBlockClosure(BlockClosure)>>ifCurtailed:
SubscriptionRegistry>>deliver:to:startingAt:
SubscriptionRegistry>>deliver:to:
SubscriptionRegistry>>deliver:
SystemAnnouncer(Announcer)>>announce:
StSindarinBytecodeDebuggerPresenter class>>showInDebugger:
[ :extension |
		| wasActivated |
		wasActivated := oldActivatedExtensions includes: extension.
		extension showInDebugger = wasActivated ifFalse: [
			extension showInDebugger: wasActivated ] ] in StDebuggerExtensionMechanismTest>>tearDown
IdentitySet(Set)>>do:
StDebuggerExtensionMechanismTest>>tearDown

https://github.com/pharo-spec/NewTools/actions/runs/7626013997/job/20771615453?pr=665

Proposed fix:

We just need to introduce an instance variable dbg to be able to clear debuggers in the tearDown. It will unsubscribe the debuggers from the extension toggle announcements