Include <stdlib.h> in test_fetch_*.cpp files (#26107)

These files use `exit()`, which requires `#include <stdlib.h>`, but
didn't have the include. These didn't error out because
`libcxx/include/math.h` used to contain `#include <stdlib.h>` but not
anymore. It was removed in
llvm/llvm-project#139586, which is included in
LLVM 21.

Author: aheejin

test/fetch/test_fetch_cached_xhr.cpp

@@ -7,6 +7,7 @@
 #include <stdio.h>
 #include <math.h>
 #include <assert.h>
+#include <stdlib.h>
 #include <emscripten/fetch.h>
 
 int result = 0;

test/fetch/test_fetch_response_headers.cpp

@@ -7,6 +7,7 @@
 #include <stdio.h>
 #include <math.h>
 #include <assert.h>
+#include <stdlib.h>
 #include <emscripten/fetch.h>
 
 int result = 1;

test/fetch/test_fetch_stream_file.cpp

@@ -7,6 +7,7 @@
 #include <stdio.h>
 #include <math.h>
 #include <assert.h>
+#include <stdlib.h>
 #include <emscripten/fetch.h>
 
 // Compute rudimentary checksum of data

test/fetch/test_fetch_to_indexeddb.cpp

@@ -7,6 +7,7 @@
 #include <stdio.h>
 #include <math.h>
 #include <assert.h>
+#include <stdlib.h>
 #include <emscripten/fetch.h>
 
 int main() {

test/fetch/test_fetch_to_memory.cpp

@@ -7,6 +7,7 @@
 #include <stdio.h>
 #include <math.h>
 #include <assert.h>
+#include <stdlib.h>
 #include <emscripten/fetch.h>
 
 int result = 1;