Fix injecting keychain instances

[crazytonyli]

The updated Blog functions all accept a keychainImplementation: KeychainAccessible = AppKeychain() parameter. But there is another way to inject test keychain: Blog.keychain. This PR makes them compatible with each other. The priority is: function parameter > Blog.keychain the stored property.

Only unit tests are affected by this PR. The production code continues to use AppKeychain.

This PR fixes test failures in #25702.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	32850
	Version	PR #25704
	Bundle ID	org.wordpress.alpha
	Commit	e3f4d63
	Installation URL	4631ag3qt4hm8

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	32850
	Version	PR #25704
	Bundle ID	com.jetpack.alpha
	Commit	e3f4d63
	Installation URL	0756g6s0pv1r8

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.