Reland "[libc] wchar string conversion functions mb to wc"

Added missing includes in the test files for null check

Author: Sriya Pratipati

libc/test/src/wchar/mbsnrtowcs_test.cpp

@@ -9,6 +9,7 @@
 #include "hdr/types/mbstate_t.h"
 #include "hdr/types/wchar_t.h"
 #include "src/__support/libc_errno.h"
+#include "src/__support/macros/null_check.h"
 #include "src/__support/wchar/mbstate.h"
 #include "src/string/memset.h"
 #include "src/wchar/mbsnrtowcs.h"

libc/test/src/wchar/mbsrtowcs_test.cpp

@@ -9,6 +9,7 @@
 #include "hdr/types/mbstate_t.h"
 #include "hdr/types/wchar_t.h"
 #include "src/__support/libc_errno.h"
+#include "src/__support/macros/null_check.h"
 #include "src/__support/wchar/mbstate.h"
 #include "src/string/memset.h"
 #include "src/wchar/mbsrtowcs.h"

libc/test/src/wchar/mbstowcs_test.cpp

@@ -8,6 +8,7 @@
 
 #include "hdr/types/wchar_t.h"
 #include "src/__support/libc_errno.h"
+#include "src/__support/macros/null_check.h"
 #include "src/wchar/mbstowcs.h"
 #include "test/UnitTest/ErrnoCheckingTest.h"
 #include "test/UnitTest/Test.h"