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

tests/http/testenv/h2o.py

@@ -241,6 +241,11 @@ class H2oServer(H2o):
         super().__init__(
             env=env, name="h2o-server", domain=env.domain1, cred_name=env.domain1
         )
+        self._docs_dir = os.path.join(self.env.gen_dir, "docs")
+
+    @property
+    def docs_dir(self):
+        return self._docs_dir
 
     def initial_start(self):
         super().initial_start()
@@ -261,11 +266,10 @@ class H2oServer(H2o):
     def write_config(self):
         creds = self.env.get_credentials(self._cred_name)
         assert creds  # convince pytype this is not None
-        doc_root = os.path.join(self.env.gen_dir, "docs")
-        self._mkpath(doc_root)
+        self._mkpath(self._docs_dir)
         self._mkpath(self._run_dir)
         # Create a simple test file
-        with open(os.path.join(doc_root, "data.json"), "w") as f:
+        with open(os.path.join(self._docs_dir, "data.json"), "w") as f:
             f.write('{"message": "Hello from h2o HTTP/3 server"}\n')
         with open(self._conf_file, "w") as fd:
             fd.write(f"""# h2o HTTP/3 server configuration
@@ -289,7 +293,7 @@ hosts:
   "{self._domain}":
     paths:
       "/":
-        file.dir: {doc_root}
+        file.dir: {self._docs_dir}
 
 http2-reprioritize-blocking-assets: ON