Check the C++ integral tests by differentiating, not by string

IntTest1 and IntTest2 compared Integrate's printed form against the literals
"x2 / 2" and "x2 / 2 + 2x". Neither says what it looks like: "x2" parses as
a variable of that name, not as a square. And neither survives Integrate
gaining its constant of integration, which is why these two were the only
failures of the twenty-four once the bindings were built against the current
source instead of a 1.4.0-preview.2 package.

The printed form was never the property under test. Both now differentiate
the antiderivative and check the difference from the integrand simplifies to
zero, which is what the rest of the suite does and what AGENTS.md asks for.

That the form was the problem rather than the mathematics is visible in the
second case: d/dx of the antiderivative of x + 2 prints as 2 + x, so even a
corrected string would have been comparing arrangements rather than values.

Measured: d/dx(x ^ 2 / 2 + C) is x, d/dx(x ^ 2 / 2 + 2 * x + C) is 2 + x, and
both differences simplify to 0. cmake is not available here, so CTest on CI
is the check.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: Rafael-SOWNet

Sources/Tests/CPPWrapperUnitTests/tests/RunTests.cpp

@@ -1,6 +1,7 @@
 #include <AngouriMath.h>
 #include <gtest/gtest.h>
 #include <vector>
+#include <string>
 
 TEST(RunTests, ParsingTest1) {
     auto src = "x / 2 + 3";
@@ -54,20 +55,25 @@ TEST(RunTests, DiffTest2) {
     EXPECT_EQ(expected.ToString(), actual.ToString());
 }
 
+// An antiderivative is checked by differentiating it back, not by comparing its printed
+// form. These two compared against the literals "x2 / 2" and "x2 / 2 + 2x", which do not
+// say what they look like -- "x2" parses as a variable of that name rather than as a
+// square -- and which could not survive Integrate gaining its constant of integration.
+// The printed form is not the property under test: d/dx of the answer is.
+static void ExpectAntiderivative(const char* integrand) {
+    AngouriMath::Entity entity = integrand;
+    auto back = entity.Integrate("x").Differentiate("x");
+    auto difference = AngouriMath::Entity(
+        "(" + back.ToString() + ") - (" + std::string(integrand) + ")");
+    EXPECT_EQ("0", difference.Simplify().ToString());
+}
+
 TEST(RunTests, IntTest1) {
-    auto src = "x";
-    AngouriMath::Entity entity = src;
-    auto actual = entity.Integrate("x");
-    auto expected = AngouriMath::Entity("x2 / 2");
-    EXPECT_EQ(expected.ToString(), actual.ToString());
+    ExpectAntiderivative("x");
 }
 
 TEST(RunTests, IntTest2) {
-    auto src = "x + 2";
-    AngouriMath::Entity entity = src;
-    auto actual = entity.Integrate("x");
-    auto expected = AngouriMath::Entity("x2 / 2 + 2x");
-    EXPECT_EQ(expected.ToString(), actual.ToString());
+    ExpectAntiderivative("x + 2");
 }
 
 TEST(RunTests, LimTest1) {