Fix MastodonSDK package tests

[aokj4ck]

Description

• ./MastodonSDK/Tests/MastodonSDKTests were previously failing to build
• This test target is runnable in ./MastodonSDK/Package.swift or through Mastodon.xcodeproj
• This change fixes the MastodonSDKTests sources
• This change adds the MastodonSDKTests target, through MastodonSDKTests.xctestplan, to the main Mastodon iOS build scheme's Test action
• When combined with Add CI step to build test target Mastodon.xcodeproj unit tests #1497 the test suite for MastodonSDK will run during pull requests to alert of any regressions in MastodonSDK sources before the PR can advance.

Please note that the tests still fail when ran for testing, but the sources compile successfully for testing.

Screenshots

Adding MastodonSDK.xctestplan to Mastodon.xcodeproj > Mastodon scheme > Testing action

Previous build warning about MastodonSDKTests missing dependencies that are fixed in this PR

[aokj4ck]

This change does not address MastodonSDK/Tests/MastodonExtensionTests/ — these test sources will need a new .testTarget declaration in MastodonSDK/Package.swift which may be suitable with this PR, or best separate in a later fix.