[path_provider] 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.

path_provider part of flutter/flutter#83357

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]

There's no native UI in path_provider, so there's no actual value I can imagine from having native UI tests for it. The goal is to have the scaffolding in place for every test we might actually want to add, but adding scaffolding that we won't ever use will just mean maintenance overhead and longer bot cycles.

[jmagman]

There's no native UI in path_provider, so there's no actual value I can imagine from having native UI tests for it. The goal is to have the scaffolding in place for every test we might actually want to add, but adding scaffolding that we won't ever use will just mean maintenance overhead and longer bot cycles.

Removed the target.

[stuartmorgan]

LGTM!