testing: update permitted number of -race goroutines
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Dmitry Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>

nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,golang/go,golang/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go