[C++] Rare bad buffer content alignment if sizeof(T) != alignof(T) (#7520)

* [C++] Add a failing unit test for #7516 (Rare bad buffer content alignment if sizeof(T) != alignof(T))

* [C++] Fix final buffer alignment when using an array of structs
* A struct can have an arbitrary size and therefore sizeof(struct) == alignof(struct)
  does not hold anymore as for value primitives.
* This patch fixes this by introducing alignment parameters to various
  CreateVector*/StartVector calls.
* Closes #7516

tests/test.cpp

@@ -20,6 +20,7 @@
 #include <string>
 
 #include "evolution_test.h"
+#include "alignment_test.h"
 #include "flatbuffers/flatbuffers.h"
 #include "flatbuffers/idl.h"
 #include "flatbuffers/minireflect.h"
@@ -1416,6 +1417,8 @@ int FlatBufferTests(const std::string &tests_data_path) {
 
   SizePrefixedTest();
 
+  AlignmentTest();
+
 #ifndef FLATBUFFERS_NO_FILE_TESTS
   ParseAndGenerateTextTest(tests_data_path, false);
   ParseAndGenerateTextTest(tests_data_path, true);