TestBlockProfile currently requires exactly five PCs in each sample.
With more aggressive inlining there may be fewer, so change this test
to use the same pattern as TestMutexProfile, which accepts one or more
PCs. With this change, this test passes when compiled with -l=4.

Change-Id: I1421a6d56c96b77111bdc671d88723a222672fd6
Reviewed-on: https://go-review.googlesource.com/41110
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Lazar <lazard@golang.org>