Adds quick_discouraged_pending_test opt-in rule

[ornithocoder]

Implements #1909.

[SwiftLintBot]

	4 Warnings
⚠️	This PR introduced a violation in Quick: /Users/distiller/project/osscheck/Quick/Tests/QuickTests/QuickTests/FunctionalTests/PendingTests.swift:17:9: warning: Quick Discouraged Pending Test Violation: Discouraged pending test. This test won't run while it's marked as pending. (quick_discouraged_pending_test)
⚠️	This PR introduced a violation in Quick: /Users/distiller/project/osscheck/Quick/Tests/QuickTests/QuickTests/FunctionalTests/PendingTests.swift:20:9: warning: Quick Discouraged Pending Test Violation: Discouraged pending test. This test won't run while it's marked as pending. (quick_discouraged_pending_test)
⚠️	This PR introduced a violation in Quick: /Users/distiller/project/osscheck/Quick/Tests/QuickTests/QuickTests/FunctionalTests/PendingTests.swift:24:13: warning: Quick Discouraged Pending Test Violation: Discouraged pending test. This test won't run while it's marked as pending. (quick_discouraged_pending_test)
⚠️	This PR introduced a violation in Quick: /Users/distiller/project/osscheck/Quick/Tests/QuickTests/QuickTests/FunctionalTests/PendingTests.swift:29:13: warning: Quick Discouraged Pending Test Violation: Discouraged pending test. This test won't run while it's marked as pending. (quick_discouraged_pending_test)

	12 Messages
📖	Linting Aerial with this PR took 0.46s vs 0.38s on master (21% slower)
📖	Linting Alamofire with this PR took 3.07s vs 2.99s on master (2% slower)
📖	Linting Firefox with this PR took 12.73s vs 12.03s on master (5% slower)
📖	Linting Kickstarter with this PR took 19.98s vs 19.33s on master (3% slower)
📖	Linting Moya with this PR took 1.43s vs 1.43s on master (0% slower)
📖	Linting Nimble with this PR took 1.81s vs 1.8s on master (0% slower)
📖	Linting Quick with this PR took 0.55s vs 0.53s on master (3% slower)
📖	Linting Realm with this PR took 3.23s vs 3.21s on master (0% slower)
📖	Linting SourceKitten with this PR took 1.05s vs 1.04s on master (0% slower)
📖	Linting Sourcery with this PR took 4.34s vs 4.26s on master (1% slower)
📖	Linting Swift with this PR took 13.59s vs 13.88s on master (2% faster)
📖	Linting WordPress with this PR took 12.06s vs 12.39s on master (2% faster)

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