Clarify -count default

Count default is only 1 when the test is not cached. 

If the test is cached it will not rerun the test. [This causes confusion because the only way to run tests without cache is to use -count=1.](#39056) According to the documentation as is `-count=1` should have no effect. Changing this CLI documentation to clarify the behavior.

Author: ScottSuarez

src/cmd/go/internal/test/test.go

@@ -208,7 +208,7 @@ control the execution of any test:
 	    (for example, -benchtime 100x).
 
 	-count n
-	    Run each test and benchmark n times (default 1).
+	    Run each test and benchmark n times (default 1 unless cached).
 	    If -cpu is set, run n times for each GOMAXPROCS value.
 	    Examples are always run once.