Exclude irrelevant tests on FreeBSD (#625)

This commit excludes current Linux-specific test cases on FreeBSD
builds. For example, the initial FreeBSD port does not support OS
Login.

google_guest_agent/oslogin_linux_test.go

diff --git a/google_guest_agent/oslogin_test.go b/google_guest_agent/oslogin_linux_test.go
similarity index 100%
rename from google_guest_agent/oslogin_test.go
rename to google_guest_agent/oslogin_linux_test.go

google_guest_agent/run/run_test.go

diff --git a/google_guest_agent/run/run_test.go b/google_guest_agent/run/run_test.go
index 70f3c90..e5a0d64 100644
--- a/google_guest_agent/run/run_test.go
+++ b/google_guest_agent/run/run_test.go
@@ -19,6 +19,7 @@
 	"fmt"
 	"os"
 	"path"
+	"runtime"
 	"strings"
 	"testing"
 	"time"
@@ -208,6 +209,10 @@
 }
 
 func TestCommandSpecSuccess(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("Skipping test: requires Linux")
+	}
+
 	type commandData struct {
 		Data string
 	}