gdb/testsuite: test get/set value of unregistered Guile parameter

When creating a parameter in Guile, you have to create it using
make-parameter and then register it with GDB with register-parameter!.
In between, it's still possible (though not documented) to set the
parameter's value.  I broke this use case by mistake while writing this
series, so thought it would be good to have a test for it.

I suppose that people could use this "feature" to give their parameter
an initial value, even though make-parameter has an initial-value
parameter for this.  Nevertheless, changing this behavior could break
some scripts, which is why I think it's important for it to be tested.

Change-Id: I5b2103e3cec0cfdcccf7ffb00eb05fed8626e66d