[libc][test] Adjust header paths in tests

Since `index_test.cpp` and `rindex_test.cpp` now reside in /strings,
adjust some header paths accordingly.

Author: Caslyn

libc/test/src/strings/index_test.cpp

@@ -6,9 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "StrchrTest.h"
+#include "test/src/string/StrchrTest.h"
 
-#include "src/string/index.h"
+#include "src/strings/index.h"
 #include "test/UnitTest/Test.h"
 
 STRCHR_TEST(Index, LIBC_NAMESPACE::index)

libc/test/src/strings/rindex_test.cpp

@@ -6,9 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "StrchrTest.h"
+#include "test/src/string/StrchrTest.h"
 
-#include "src/string/rindex.h"
+#include "src/strings/rindex.h"
 #include "test/UnitTest/Test.h"
 
 STRRCHR_TEST(Rindex, LIBC_NAMESPACE::rindex)