[libc] Add a functioning realloc for hermetic tests.

[jhuber6]

You missed the char_vector_test in __support.

[sivachandra]

Enable hermetic char_vector_test.

libc/test/UnitTest/HermeticTestUtils.cpp

void *malloc(size_t s) {
   s = ((s + ALIGNMENT - 1) / ALIGNMENT) * ALIGNMENT;
   void *mem = ptr;
   ptr += s;
   return static_cast<uint64_t>(ptr - memory) >= MEMORY_SIZE ? nullptr : mem;
 }
 
 void free(void *) {}
 
-void *realloc(void *ptr, size_t s) {
-  free(ptr);
+void *realloc(void *mem, size_t s) {
+  if (mem == nullptr)
     return malloc(s);
+  uint8_t *newmem = reinterpret_cast<uint8_t *>(malloc(s));
+  if (newmem == nullptr)
+    return nullptr;
+  uint8_t *oldmem = reinterpret_cast<uint8_t *>(mem);
+  // We use a simple for loop to copy the data over.
+  // If |s| is less the previous alloc size, the copy works as expected.
+  // If |s| is greater than the previous alloc size, then garbage is copied
+  // over to the additional part in the new memory block.
+  for (size_t i = 0; i < s; ++i)
+    newmem[i] = oldmem[i];
+  return newmem;
 }
 
 // The unit test framework uses pure virtual functions. Since hermetic tests
 // cannot depend C++ runtime libraries, implement dummy functions to support
 // the virtual function runtime.
 void __cxa_pure_virtual() {
   // A pure virtual being called is an error so we just trap.
   __builtin_trap();

libc/test/src/__support/CMakeLists.txt

add_libc_test(
   SRCS
     fixedvector_test.cpp
   DEPENDS
     libc.src.__support.fixedvector
 )
 
 add_libc_test(
   char_vector_test
-  # This test relies on 'realloc' which is not implemented for hermetic tests.
-  UNIT_TEST_ONLY
   SUITE
     libc-support-tests
   SRCS
     char_vector_test.cpp
   DEPENDS
     libc.src.__support.char_vector
 )
 

libc/test/src/__support/CPP/CMakeLists.txt

add_libc_test(
   SRCS
     span_test.cpp
   DEPENDS
     libc.src.__support.CPP.span
 )
 
 add_libc_test(
   string_test
-  # This test relies on 'realloc' which is not implemented for hermetic tests.
-  UNIT_TEST_ONLY
   SUITE
     libc-cpp-utils-tests
   SRCS
     string_test.cpp
   DEPENDS
   libc.src.__support.CPP.string
   libc.src.__support.CPP.string_view
 )