Fix Single Equality Simplification

[rcosta358]

There was a bug in the expression simplification logic where an expression, e.g., x == 1 was being simplified to true, since the variable x was replaced with its known value 1, resulting in the tautology 1 == 1 which would then get folded to true.

This would then result in error messages such as "true is not a subtype of #x_0 < 0".

This was fixed in the VariableResolver by checking if the expression we want to extract the variables from is a single top-level equality and, in this case, don't add x -> 1 to the variable map.

Also added some tests for the VariableResolver and an extra test case for this case in the ExpressionSimplifierTest.

[CatarinaGamboa]

lgtm