[release-branch.go1.15] syscall: use MustHaveExec in TestExec

For #41702
For #41704

Change-Id: Ib2b15e52aa1fef2f5e644b316c726150252fa9f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/262738


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit 11cfb48d)
Reviewed-on: https://go-review.googlesource.com/c/go/+/264020