Add a few unit tests for HttpFile

This just tests the success case. Failure cases will need additional
work.

test/data/README.rst

@@ -36,3 +36,8 @@ rfi_mask_ranges_is_group.hdf5
     There is a group called ``ranges`` rather than a dataset.
 wrong_extension.blah
     Copy of rfi_mask_ranges.hdf5 with a different extension.
+
+Other
+-----
+all_bytes.bin
+    Byte values running from 0 to 255.

test/test_fetch.py

@@ -18,6 +18,7 @@
 
 import contextlib
 import hashlib
+import io
 
 import h5py
 import pytest
@@ -28,6 +29,46 @@
 from test_utils import get_data, get_data_url, get_file_url, DummyModel
 
 
+@pytest.fixture
+def http_file(web_server):
+    with requests.Session() as session:
+        with fetch.HttpFile(session, web_server('all_bytes.bin')) as file:
+            yield file
+
+
+def test_http_file_seek_tell(http_file):
+    assert http_file.tell() == 0
+    http_file.seek(10, io.SEEK_SET)
+    assert http_file.tell() == 10
+    assert http_file.read(2) == b'\x0A\x0B'
+    assert http_file.tell() == 12
+    http_file.seek(5)
+    assert http_file.tell() == 5
+    http_file.seek(5, io.SEEK_CUR)
+    assert http_file.tell() == 10
+    http_file.seek(-5, io.SEEK_CUR)
+    assert http_file.tell() == 5
+    http_file.seek(-10, io.SEEK_END)
+    assert http_file.tell() == 246
+
+
+def test_http_file_close(http_file):
+    assert not http_file.closed
+    http_file.close()
+    assert http_file.closed
+    http_file.close()
+    assert http_file.closed
+
+
+def test_http_file_read(http_file):
+    assert http_file.read(2) == b'\x00\x01'
+    assert http_file.read(3) == b'\x02\x03\x04'
+    http_file.seek(-2, io.SEEK_END)
+    # Short read at end of file
+    assert http_file.read(4) == b'\xFE\xFF'
+    assert http_file.tell() == 256
+
+
 @pytest.mark.parametrize('use_file', [True, False])
 @pytest.mark.parametrize('filename', ['rfi_mask_ranges.hdf5', 'direct.alias', 'indirect.alias'])
 def test_fetch_model_simple(use_file, filename, web_server) -> None: