Add more Functions tests

[sgrekhov]

Add more tests for 9. Functions

[sgrekhov]

There are no tests for the following statement

• of the form => e or the form async => e, which both return the value
of the expression e as if by a return e. The other modifiers do not apply
here, because they apply only to generators, discussed below. Generators are
not allowed to explicitly return anything, objects are added to the generated
stream or iterable using yield or yield*. Let T be the declared return type
of the function that has this body. It is a compile-time error if one of the
following conditions hold:
– The function is synchronous, T is not void, and it would have been a
compile-time error to declare the function with the body { return e;
} rather than => e