Add a test helper for creating a dummy Credentials (#2145)

* Add convenience method `Credentials::for_tests`

This commit addresses https://github.com/awslabs/smithy-rs/pull/2122#discussion_r1056499862.

We considered using `#[cfg(test)]` for it but the attribute only works
within the defining crate. To work around it, we have added `#[doc(hidden)]`
to it to prevent the method from appearing in the public doc, though still
accessible in the source code.

* Make AWS runtime crates use test credentials helper

This commit updates the test code in the AWS runtime crates to use
`Credentials::for_tests` wherever applicable. We intentionally did
not replace the following `Credentials::new` occurrences:
* those in public rustdoc
* those that need to configure expiration time

* Make EndpointsCredentialsTest use test credentials helper

This commit replaces `Credentials::new` with `Credentials::for_tests` in
the unit test for EndpointsCredentialsTest.

* Make SDK integration tests use test credentials helper

This commit replaces `Credentials::new` with `Credentials::for_tests` in
the test code in the SDK integration tests wherever applicable.

* Call `Credentials::for_tests` directly in calling `credentials_provider`

* Feature gate `Credentials::for_tests` behind "test-util"

This commit addresses https://github.com/awslabs/smithy-rs/pull/2145#discussion_r1059437196

* Use `Credentials::for_tests` rather than `new`

This commit addresses https://github.com/awslabs/smithy-rs/pull/2145#discussion_r1059437435



Co-authored-by: Yuki Saito <awsaito@amazon.com>