Move Swift TestWaiter into module

[shepazon]

Existing Example Update

The submitter has:

• [X] Confirmed that the correct copyright is included in all files.
• [ ] Major code changes have been reviewed, and the submitter has incorporated review comments.
• [ ] Changed or added comments and strings have been reviewed, and the submitter has incorporated any and all suggested edits.

Description of Changes

This update moves the TestWaiter type defined previously in the S3 Basics example into the SwiftUtilities module so that it can be reused by other example tests that need it.

Since this is a change affecting only code tests, there's no test build of the doc library associated with this.

Please describe the changes you have made here.

• [X] I have tested my changes and created unit tests for new code paths.
• [ ] Changes have been reviewed, and all reviewer comments have been incorporated.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[shepazon]

Actual changes here are pretty minimal. Things to look at are basically:

• Are the tests for the TestWaiter type that have now been added to the module covering well enough (@beqqrry-aws)?
• Are the comments in the tests written well (@lkdavies)? (There've been no text changes anywhere else)
• Probably @Laren-AWS has little if anything to do here since the only code change is that the two init() functions in the TestWaiter struct have been merged into one with defaults, and the init() function and the TestWaiter structure have been made public to be available to module users.