The SigTest should check compile-time constants in source mode.

Compile-Time constants affects source compatibility.
If a field, which is a compile-time constant, is changed to become
a non-compile-time constant, them some sources will not compile against
the new version, because compile-time constants can be used in
"case <compile-time constant>:" expressions but regular fields can not.

For example the following class can be compiled:

class Other {
    public static final int test = 1;
    public static int getInt() {
        return 1;
    }
}
class Test {
    public static final int test = 1;
    public static void main(String[] argv) {
       switch(Other.getInt()) {
       case Other.test: System.out.println("OK"); break;
       }
    }
}

But changing of the line
    public static final int test = 1;
by
    public static final int test = getInt();

produces the following compile-time error.

  Test.java:11: constant expression required
         case Other.test: System.out.println("OK"); break;
                 ^
  1 error

The changing of the values does not break source compatibility if the
fields is still compile-time constant. But we have to check values because
they are part of the specification.