cmd/internal/pkgpath: use MustHaveExec in TestToSymbolFunc

cmd/internal/pkgpath: use MustHaveExec in TestToSymbolFunc

ToSymbolFunc executes a command using exec.Command.

Change-Id: Ic6c274bdc23050d021ec98ba67748338ac34f12c
Reviewed-on: https://go-review.googlesource.com/c/go/+/435236
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>