Tests: Add unit tests for property redefinition

From reading `Source/cmDefinePropertyCommand.cxx` and
`Source/cmPropertyDefinition.cxx`, attempts to use `define_property()`
to redefine an existing property will be silently ignored.
Once a property is defined, it cannot be redefined.

Add some unit tests to confirm this behavior of `define_property()`
remains as expected based on the current implementation.