testing: update permitted number of -race goroutines

The value 8128 appears to be correct as of 2021-03-23. The value is
determined by the value of kMaxTid currently at

https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/tsan/rtl/tsan_defs.h#L68

Fixes #45193

Change-Id: If4a30d7dbebd6775bede42f565dc8a741b8b036c
Reviewed-on: https://go-review.googlesource.com/c/go/+/304254


Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>