test sharding runs Example tests in each shard

### What version of rules_go are you using?

0.34

### What version of Bazel are you using?

5.1.0

### Does this issue reproduce with the latest releases of all the above?

Yes

### What operating system and processor architecture are you using?

macOS

### What did you do?

I use sharded testing. Some of the tests are `Example` tests. They seem to run in every shard.

### What did you expect to see?

I'd expect to see the Example run in only one shard.

### What did you see instead?

It's run in every shard.

Reproduce the sharded test behavior with rules_go 0.34 and Bazel 5.1.0 on macOS, using a test suite that includes Example tests. Trace the test-sharding implementation and its handling of Example tests; done when each Example runs in only one shard rather than every shard.

Written by the indexing model from the issue text.