testing: document -race goroutine limits

-race sets a hard cap of 8,192, which is easily hit while testing.

Fixes #23611

Change-Id: I0f720ec39c82c2194a485d437d6373f4bdc8a9c1
Reviewed-on: https://go-review.googlesource.com/103160Reviewed-by: Rob Pike <r@golang.org>

src/testing/testing.go

@@ -178,6 +178,9 @@
 //         }
 //     }
 //
+// The race detector kills the program if it exceeds 8192 concurrent goroutines,
+// so use care when running parallel tests with the -race flag set.
+//
 // Run does not return until parallel subtests have completed, providing a way
 // to clean up after a group of parallel tests:
 //