Support hidden environment variables in tests

[mccoyp]

Description

Resolves #44047. The EnvironmentVariableLoader currently passes all environment variable values to tests as plain strings. A consequence of this is that variable values can be exposed in pytest outputs, since pytest logs the test inputs and local variables upon failure. While this is helpful for debugging, it can be problematic when these variables are used to store secrets (e.g. account keys).

This PR adds a hide_secrets kwarg to the EnvironmentVariableLoader, which allows the user to specify which environment variables should be considered sensitive/secret. These variables will have their values wrapped in an EnvironmentVariable instance that has an overridden, redacted __str__ implementation to hide secrets in logs. To get the value during test execution, just use the .secret attribute wherever the value would have been used directly before.

This PR also migrates azure-storage-file-share tests to hide account keys.

As a draft, documentation in tests.md is pending. Storage tests have been confirmed locally to work in playback, but they still need to be tested in live mode.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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