Add native unit tests to iOS and macOS templates

[stuartmorgan]

Adds an example native unit test to the templates for iOS and macOS, matching the format we use for our 1P plugin example app unit tests. Once these have been added for all platforms+languages, they will be documented on a new plugin development page to explain their use.

For plugin example apps, the test shows an example of an actual plugin test. For standard apps, the test is an empty placeholder (similar to what Xcode would create when adding a unit test target). Although most people aren't likely to use the unit test targets in apps currently, having it be unconditional makes it much easier to maintain the project.

Also updates the Swift plugin example itself to show handling methods by name, matching the macOS Swift plugin example, since it makes it provides a better foundation for real-world plugin development (where there is very often more than one method name).

Part of #82458

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, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[stuartmorgan]

This doesn't include macOS yet since I wanted to get your thoughts on this whole approach before replicating it (especially since the manual pxproj surgery is tedious).

[jmagman]

Suggest also adding a runXcodeTests call for iOS and macOS plugin example projects around here:

flutter/dev/devicelab/lib/tasks/plugin_tests.dart

Lines 61 to 63 in fc3571e

	if (runFlutterTest) {
	await plugin.test();
	}

See example

flutter/dev/devicelab/bin/tasks/native_platform_view_ui_tests_ios.dart

Lines 49 to 53 in 8e2333d

	if (!await runXcodeTests(
	platformDirectory: path.join(projectDirectory, 'ios'),
	destination: 'id=${device.deviceId}',
	testName: 'native_platform_view_ui_tests_ios',
	)) {

Can use testWithNewIOSSimulator deviceId to run tests on the simulator, see

flutter/dev/devicelab/bin/tasks/module_test_ios.dart

Line 474 in 7c8e171

await testWithNewIOSSimulator('TestAdd2AppSim', (String deviceId) async {

[jmagman]

03:49 +167 ~1 -1: test/commands.shard/permeable/create_test.dart: plugin includes native unit tests [E]                                                                                                
  Expected: a file system entity that exists
    Actual: LocalFile:<LocalFile: '/b/s/w/ir/x/t/flutter_tools.CSBNSE/flutter_tools_create_test.OVGLNS/flutter_project/example/ios/RunnerTests/FlutterProjectPluginTest.swift'>
     Which: does not exist

[stuartmorgan]

Sorry, I had roughed out the test but not actually run it yet, then forgot it was in there when I posted the draft for feedback.

[stuartmorgan]

Updated with everything but the device lab tests; I'll update again with those when I have a chance.

[jmagman]

What you have LGTM, will review again when the devicelab tests are added.

[stuartmorgan]

Looks like the devicelab tests are working, thanks for all the pointers!

[jmagman]

LGTM!