This PR refactors this slider test so that

• The test is split into multiple tests in a single group for better readability.
• A single expectValueIndicator tests both the dragged state and the released state, for better readability.
• The widget state is reset at the end of expectValueIndicator to avoid state leak...
• ... which revealed a test case that has been written incorrectly.
• ShowValueIndicator.alwaysVisible's doc is corrected to reflect its actual behavior.

This PR is split from, and is a prerequisite of, #182991.

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.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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