test_measurement_operator takes too long on arm64

[olebole]

Hi,

since casacore is now available for the arm64 platform, we also tried to build purify there. The build fails due to a timeout in test_measurement_operator (see Debian bug #867273). According to a comment from @egrimley, this may have to do with the data type used in the test:

I think that test takes a long time because it is using long double, which on arm64 has 128 bits and is implemented in software. Possible things to do:

• Change the default type (however and wherever it is defined) from "long double" to "double" on arm64, and perhaps other architectures.

• Get that particular test to use 64-bit floats.

• Get that test to generate some output as it runs so that it does not seem to be inactive.

Could you give me some advice what is the best to do here?