test: add new https_serve_directory() and test certs

This commit adds a new `https_serve_directory()` test helper
and some custom self-signed and worthless certs that are used
during testing. They are not dynamically generated to avoid the
extra compuation time during tests (but they could be).

Generated via:
```
$ openssl req -new -newkey rsa:2048  -nodes -x509  \
   -subj "/C=DE/ST=Berlin/L=Berlin/O=Org/CN=localhost"   \
   -keyout "key1.pem" -out "cert1.pem"
```

This will allow us to test `https` download URLs as well in e.g.
the curl source.

test/mod/test_testutil_net.py

@@ -0,0 +1,39 @@
+import os.path
+import pathlib
+import subprocess
+
+from osbuild.testutil import make_fake_tree
+from osbuild.testutil.net import http_serve_directory, https_serve_directory
+
+
+def test_http_serve_directory_smoke(tmp_path):
+    make_fake_tree(tmp_path, {
+        "file1": "file1 content",
+        "dir1/file2": "file2 content",
+    })
+    with http_serve_directory(tmp_path) as httpd:
+        output = subprocess.check_output(
+            ["curl", f"http://localhost:{httpd.server_port}/file1"])
+        assert output == b"file1 content"
+        output = subprocess.check_output(
+            ["curl", f"http://localhost:{httpd.server_port}/dir1/file2"])
+        assert output == b"file2 content"
+
+
+def test_https_serve_directory_smoke(tmp_path):
+    make_fake_tree(tmp_path, {
+        "file1": "file1 content",
+    })
+    cert_dir = pathlib.Path(__file__).parent.parent / "data/certs"
+    cacertfile = cert_dir / "cert1.pem"
+    assert cacertfile.exists()
+    keyfile = cert_dir / "key1.pem"
+    assert keyfile.exists()
+
+    with https_serve_directory(tmp_path, cacertfile, keyfile) as httpd:
+        output = subprocess.check_output(
+            ["curl",
+             "--cacert", os.fspath(cacertfile),
+             f"https://localhost:{httpd.server_port}/file1"],
+        )
+        assert output == b"file1 content"