Improve testing of the codegen system

- Move codegen tests to a sub directory
- Add tests for all functions of Expression, ExpressionRef, and ExpressionGraph
- Respect dependencies during tests: The ExpressionGraph test doesn't
  use ExpressionRef anymore.

The new tests revealed a few bugs so the following changes were made:

- Expression::MakeNop now resets the current expression with the default
  constructed NOP expression
- ExpressionGraph::Insert now updates the lhs_id the same way as
  InsertBack()

Change-Id: I6a18925c1e4d972c29ec1219f2073b4eaf2df737

include/ceres/codegen/internal/expression.h

@@ -249,6 +249,7 @@ enum class ExpressionType {
 // ExpressionGraph (see expression_graph.h).
 class Expression {
  public:
+  // Creates a NOP expression.
   Expression() = default;
 
   Expression(ExpressionType type,

include/ceres/codegen/internal/expression_graph.h

@@ -53,6 +53,9 @@ class ExpressionGraph {
   bool DependsOn(ExpressionId A, ExpressionId B) const;
 
   bool operator==(const ExpressionGraph& other) const;
+  bool operator!=(const ExpressionGraph& other) const {
+    return !(*this == other);
+  }
 
   Expression& ExpressionForId(ExpressionId id) { return expressions_[id]; }
   const Expression& ExpressionForId(ExpressionId id) const {