plugin: using coverpkg when testing results in plugin version mismatch

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

1.9.2

Does this issue reproduce with the latest release?

yes - 1.9.2 is the latest, stable release

What operating system and processor architecture are you using (go env)?

GOARCH="amd64" GOBIN="" GOEXE="" GOHOSTARCH="amd64" GOHOSTOS="linux" GOOS="linux" GOPATH="/home/vagrant/pluginbugs" GORACE="" GOROOT="/usr/local/go-1.9.2" GOTOOLDIR="/usr/local/go-1.9.2/pkg/tool/linux_amd64" GCCGO="gccgo" CC="gcc" GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build917652042=/tmp/go-build -gno-record-gcc-switches" CXX="g++" CGO_ENABLED="1" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config"

What did you do?

https://github.com/jdef/plugins1/blob/master/README.md

What did you expect to see?

• all tests pass successfully

What did you see instead?

• the go test invocation using -coverpkg fails with "plugin was built with a different version of package..." message