Make FunctionMetadata test more robust.

[aditi-pandit]

FunctionMetadataTest https://github.com/prestodb/presto/blob/master/presto-native-execution/presto_cpp/main/functions/tests/FunctionMetadataTest.cpp fails when adding new function signatures.

Make the test more robust to signature addition by changing the logic in testFunction to ensure that the existing signatures remain and they have the fields expected in the json instead of testing specific values.

[aditi-pandit]

@pratikpugalia @amitkdutta @kgpai