cmd/go/internal/test: clarify -count default

[ScottSuarez]

Count default is only 1 when the test is not cached.
If the test is cached it will not rerun the test.

This documentation causes confusion because the only way to run tests without cache is to use -count=1. According to the documentation as is -count=1 should have no effect. Changing this CLI documentation to clarify the behavior.

ref: #39056
resolves: #39056

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Patch Set 2: Code-Review-1

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[ScottSuarez]

closing this after comment inline clarified with explanation for current description.