Disable warning messages of testing

With new version dtc, running test cases in libufdt will get
some warning messages when compiling overlay DT, such as:

    Warning (unit_address_vs_reg): Node /fragment@0 has a unit name,
    but no reg property

Avoid to noise the result of testing, add parameter -qq to disable
these warning messages in unit testing.

Bug: 33908117
Test: run test scripts (see libufdt/tests/README)
Change-Id: I874562cc00e1cb49c3417ea83f09925c77939207