allow arbitrary declarations in language tests

goal is to move towards eliminating StandardDeclarations (instead
making each test declare, itself, only what it needs)
language tests: make StandardDeclarations optional

StandardDeclarations now only applies to TestCommands, and only
if the test does not make any custom declarations of its own

(this constitutes progress towards eliminating StandardDeclarations
entirely)
eliminate StandardDeclarations from language tests

all tests now themselves declare only what they need