[shared_preferences] Add iOS unit tests

[jmagman]

Add RunnerTests XCTest iOS target to example app that creates the plugin and asserts it's not nil, proving linking is set up correctly.

shared_preferences part of flutter/flutter#83357

https://cirrus-ci.com/task/5671640303403008?logs=xctest#L1544

Test Suite 'SharedPreferencesTests' passed at 2021-06-03 18:31:54.382.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.003 (0.004) seconds
Test Suite 'RunnerTests.xctest' passed at 2021-06-03 18:31:54.383.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.003 (0.007) seconds

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides]. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool format)
• I signed the [CLA].
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[stuartmorgan-g]

LGTM