tests/http: fix HTTP/3 proxy pytest failures with h2o

Fix pytest failures in HTTP/3 proxy tests when h2o is not installed,
misconfigured, or fails to start at runtime.

This prevents:
- FileNotFoundError when h2o document root does not exist
- Fixture setup errors when h2o is configured but cannot start
- Unused test data file creation when h2o is absent or broken
- CI aborts on systems where h2o exists but is not runnable

Bug: https://github.com/curl/curl/pull/21789#issuecomment-4559098879
Bug: https://github.com/curl/curl/pull/21789#issuecomment-4559161907

Follow-up to e78b1b3ecc #21153

Closes #21791

tests/http/testenv/env.py

@@ -909,6 +909,7 @@ class Env:
     ) -> str:
         if line_length < 11:
             raise RuntimeError("line_length less than 11 not supported")
+        os.makedirs(indir, exist_ok=True)
         fpath = os.path.join(indir, fname)
         s10 = "0123456789"
         s = round((line_length / 10) + 1) * s10