Added unittest for the test case in to check the performOperation function's ability to perform subtraction. Includes arranging operands, performing subtraction, and asserting the expected result.

src/test/c/test_taschenrechner.c

@@ -168,6 +168,16 @@ void test_performOperation_Addition(void) {
     TEST_ASSERT_EQUAL_INT(8, result);
 }
 
+// Test case for subtraction
+void test_performOperation_Subtraction(void) {
+    // Arrange
+    int result;
+
+    // Act
+    result = performOperation(10, '-', 3);
 
+    // Assert
+    TEST_ASSERT_EQUAL_INT(7, result);
+}
 
 #endif // TEST