Update instructions on running integration_test with the flutter driver

[mvanbeusekom]

Improve the documentation on running integration tests using the Flutter driver in the CONTRIBUTING.md file. Currently the instructions mention to run the following command:

flutter drive test_driver/<name_of_plugin_test>.dart

This will fail with the following error:

Test file not found: /Users/maurits/Projects/Baseflow/Internal/Flutter/plugins/packages/in_app_purchase/in_app_purchase_android/example/test_driver/test/integration_test_test.dart

To correctly execute the integration tests locally, one should specify the --driver and --target flags correctly. The documentation now shows the correct example.

List which issues are fixed by this PR. You must list at least one issue.

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

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.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[cyanglaz]

LGTM

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• The status or check suite build-ipas+drive-examples has failed. Please fix the issues identified (or deflake) before re-applying this label.
• The status or check suite build_all_plugins_macos has failed. Please fix the issues identified (or deflake) before re-applying this label.
• The status or check suite build-macos+drive-examples has failed. Please fix the issues identified (or deflake) before re-applying this label.
• This commit has empty status or empty checks. Please check the Google CLA status is present and Flutter Dashboard application has multiple checks.

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• This commit has empty status or empty checks. Please check the Google CLA status is present and Flutter Dashboard application has multiple checks.

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• This commit has empty status or empty checks. Please check the Google CLA status is present and Flutter Dashboard application has multiple checks.