[release-branch.go1.14] syscall: use MustHaveExec in TestExec
For #41702
For #41703
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
11cfb48df192c14d185c1cfcaad1ba3e7b84c807)
Reviewed-on: https://go-review.googlesource.com/c/go/+/264021