testing: synchronize writes to the root's Writer

Prior to this change it was possible to see interleaved messages:
<<<
=== RUN   Test/LongLongLongLongName48
=== RUN   Test/LongLon=== RUN   Test/LongLongLongLongName50
gLongLongName49
=== RUN   Test/LongLongLongLongName51
>>>

This change fixes it such that you see:
<<<
=== RUN   Test/LongLongLongLongName48
=== RUN   Test/LongLongLongLongName49
=== RUN   Test/LongLongLongLongName50
=== RUN   Test/LongLongLongLongName51
>>>

Fixes #18741

Change-Id: I2529d724065dc65b3e9eb3d7cbeeda82a2d0cfd4
Reviewed-on: https://go-review.googlesource.com/35556


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>