Rename "testing" feature to "for-internal-testing-only"

[joshtriplett]

Rather than having a feature named "testing" that external users might
be tempted to incorrectly use (and a detailed comment discouraging them,
which people may or may not see), rename the feature to
"for-internal-testing-only" so that nobody gets any wrong ideas about
whether it's meant for them.

[joshtriplett]

Sorry, I sent this for the main branch before realizing that sled 1.0 development is on a different branch. Doesn't look like this is needed on the latter. If you're not looking for changes on the main branch, feel free to close this.

[spacejam]

Thanks, Josh :) While the 1.0 branch doesn't have this yet, it will receive some amount of test-only event tracking / expensive assertion verifiers that will use a feature, and I like using the feature name you proposed, so I'll merge it into main and port it to 1.0 when adding the new verification layers.