cmd/ocagent/plugins: best case Go binary path resolution

Avoid Travis CI failures due to building plugins and running
binary in
    https://travis-ci.org/census-instrumentation/opencensus-service/builds/438157975

by trying:
1) runtime.GOROOT .join "go*"
2) Looking up Go from the current path

Author: odeke-em

cmd/ocagent/plugins/plugins_test.go

@@ -26,6 +26,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"testing"
 	"time"
 
@@ -43,7 +44,8 @@ func TestCreatePlugin(t *testing.T) {
 	defer os.RemoveAll(tmpDir)
 
 	pluginPath := filepath.Join(tmpDir, "sample.so")
-	output, err := exec.Command("go", "build", "-buildmode=plugin", "-o", pluginPath, "./testdata/sample_plugin.go").CombinedOutput()
+	cmd := exec.Command(properGoBinaryPath(), "build", "-buildmode=plugin", "-o", pluginPath, "./testdata/sample_plugin.go")
+	output, err := cmd.CombinedOutput()
 	if err != nil {
 		t.Fatalf("Failed to compile and create shared object file %q: %v\n%s", pluginPath, err, output)
 	}
@@ -79,3 +81,23 @@ counter:
 		t.Errorf("Unexpected output: %s", g)
 	}
 }
+
+// This helper function is necessary to ensure that we use
+// the same Go binary to compile plugins as well as to run
+// this test.
+// Otherwise we'll run into such failed tests:
+//      https://travis-ci.org/census-instrumentation/opencensus-service/builds/438157975
+func properGoBinaryPath() string {
+	goSuffix := "go"
+	if runtime.GOOS == "windows" {
+		goSuffix += ".exe"
+	}
+	// Firstly check if we are using $GOROOT/bin/go*
+	goBinPath := filepath.Join(runtime.GOROOT(), "bin", goSuffix)
+	if _, err := os.Stat(goBinPath); err == nil {
+		return goBinPath
+	}
+	// If that has failed, now trying looking it from the environment
+	binPath, _ := exec.LookPath(goSuffix)
+	return binPath
+}