Fix MakeFileBody/MakeFileProvider tests on Windows

These tests wrote to a hardcoded "/tmp/" path which does not exist on
Windows, causing the file write to silently fail and the subsequent
make_file_body / make_file_provider call to return zero-sized data.
Use a relative path under the test working directory instead so the
test runs identically on every platform.

Also dump the shard log when a shard's process exits non-zero even
when the gtest summary appears clean (e.g. sanitizer report after
the suite, or assertion-based abort) — previously such failures were
detected only via overall rc and showed no diagnostic output.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/test.cc

@@ -12276,7 +12276,7 @@ TEST(MultipartFormDataTest, ManyItemsEndToEnd) {
 TEST(MultipartFormDataTest, MakeFileProvider) {
   // Verify make_file_provider sends a file's contents correctly.
   const std::string file_content(4096, 'Z');
-  const std::string tmp_path = "/tmp/httplib_test_make_file_provider.bin";
+  const std::string tmp_path = "./httplib_test_make_file_provider.bin";
   {
     std::ofstream ofs(tmp_path, std::ios::binary);
     ofs.write(file_content.data(),
@@ -12331,7 +12331,7 @@ TEST(MultipartFormDataTest, MakeFileProvider) {
 
 TEST(MakeFileBodyTest, Basic) {
   const std::string file_content(4096, 'Z');
-  const std::string tmp_path = "/tmp/httplib_test_make_file_body.bin";
+  const std::string tmp_path = "./httplib_test_make_file_body.bin";
   {
     std::ofstream ofs(tmp_path, std::ios::binary);
     ofs.write(file_content.data(),