Spanner: remove junit from compile dependencies (#4334)

Author: mziccard

google-cloud-clients/google-cloud-spanner/pom.xml

@@ -133,12 +133,13 @@
       <groupId>joda-time</groupId>
       <artifactId>joda-time</artifactId>
     </dependency>
+
+    <!-- Test dependencies -->
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>
+      <scope>test</scope>
     </dependency>
-
-    <!-- Test dependencies -->
     <dependency>
       <groupId>com.google.truth</groupId>
       <artifactId>truth</artifactId>