The problem with a single serie of tests is that it will only be valid for the radar altimeter type installed in the testing aircraft, and for the same antenna fit.
The lack of radar altimeter specifications results in having different electronic characteristics for each altimeter type.
I believe that this issue will only be overcome if each altimeter maker provides the authorities with the results of standard tests on the interference immunity of their device : standard size and gain of the supported antennas, characteristics of their band filter, saturation curve of their first amplification stage vs frequency, supported in-band interference and noise level, etc.