make.go: skip codesigning when testing an existing build

[ardnew]

When running make to re-test an existing build, the test target fails with the following error (on macOS):

$ make
Testing default backend
=== RUN   TestBuild
    dlv_test.go:92: makefile error: exit status 1
        output go build -ldflags "-extldflags -static" -tags=macnative "-ldflags=-s -X main.Build=88380654feb85c213d2bb60532d95be8d562c86e" github.com/go-delve/delve/cmd/dlv
        codesign -s dlv-cert ./dlv
        ./dlv: is already signed
        exit status 1

This PR adds a check to _scripts/make.go to simply skip the codesigning step if the codesign --verify command reports the executable is already signed with a valid certificate.

[aarzilli]

LGTM

[derekparker]

lgtm