Testing needs instrumentation

Right, it can be done but the outcome of adding such a test may be worse than without a test.

All serious RNG implementations use some kind of hash function before returning the random
bytes, thus you can't run statistical tests on the output. To run tests you need to instrument
the code.  That very instrumentation is a source of bugs and thus it should be avoided. In
fact, OpenSSL was recently trapped by such a bug due to extra code added for the FIPS
validation of OpenSSL.