Restore GOPATH-mode get (revert de4d503)

src/cmd/go/testdata/script/get_goroot.txt

@@ -0,0 +1,53 @@
+[!net:github.com] skip
+env GO111MODULE=off
+
+# Issue 4186. go get cannot be used to download packages to $GOROOT.
+# Test that without GOPATH set, go get should fail.
+
+# Fails because GOROOT=GOPATH
+env GOPATH=$WORK/tmp
+env GOROOT=$WORK/tmp
+! go get -d github.com/golang/example/hello
+stderr 'warning: GOPATH set to GOROOT'
+stderr '\$GOPATH must not be set to \$GOROOT'
+
+# Fails because GOROOT=GOPATH after cleaning.
+env GOPATH=$WORK/tmp/
+env GOROOT=$WORK/tmp
+! go get -d github.com/golang/example/hello
+stderr 'warning: GOPATH set to GOROOT'
+stderr '\$GOPATH must not be set to \$GOROOT'
+
+env GOPATH=$WORK/tmp
+env GOROOT=$WORK/tmp/
+! go get -d github.com/golang/example/hello
+stderr 'warning: GOPATH set to GOROOT'
+stderr '\$GOPATH must not be set to \$GOROOT'
+
+# Make a home directory
+mkdir $WORK/home/go
+
+# Fails because GOROOT=$HOME/go so default GOPATH unset.
+[GOOS:windows] env USERPROFILE=$WORK/home
+[GOOS:plan9] env home=$WORK/home
+[!GOOS:windows] [!GOOS:plan9] env HOME=$WORK/home
+env GOPATH=
+env GOROOT=$WORK/home/go
+! go get -d github.com/golang/example/hello
+stderr '\$GOPATH not set'
+
+[GOOS:windows] env USERPROFILE=$WORK/home/
+[GOOS:plan9] env home=$WORK/home/
+[!GOOS:windows] [!GOOS:plan9] env HOME=$WORK/home/
+env GOPATH=
+env GOROOT=$WORK/home/go
+! go get -d github.com/golang/example/hello
+stderr '\$GOPATH not set'
+
+[GOOS:windows] env USERPROFILE=$WORK/home
+[GOOS:plan9] env home=$WORK/home
+[!GOOS:windows] [!GOOS:plan9] env HOME=$WORK/home
+env GOPATH=
+env GOROOT=$WORK/home/go/
+! go get -d github.com/golang/example/hello
+stderr '\$GOPATH not set'