testutil: add new mock_command context manager

The new `testutil.mock_command` context manager can be used to
mock commands in PATH and replace them with arbitrary shell
scripts. This is useful in testing to e.g. simulate exact error
conditions that would be hard to trigger otherwise or to replace
long running commands with faked results.

Example:
```
fake_cmd = textwrap.dedent("""\
do-something
""")
with mock_command("some-cmd", fake_cmd):
   your_code
```

osbuild/testutil/__init__.py

@@ -1,10 +1,12 @@
 """
 Test related utilities
 """
+import contextlib
 import os
 import pathlib
 import re
 import shutil
+import tempfile
 
 
 def has_executable(executable: str) -> bool:
@@ -57,3 +59,21 @@ def assert_jsonschema_error_contains(res, expected_err, expected_num_errs=None):
         def finder(s): return expected_err in s  # pylint: disable=C0321
     assert any(finder(err_msg)
                for err_msg in err_msgs), f"{expected_err} not found in {err_msgs}"
+
+
+@contextlib.contextmanager
+def mock_command(cmd_name: str, script: str):
+    """
+    mock_command creates a mocked binary with the given :cmd_name: and :script:
+    content. This is useful to e.g. mock errors from binaries.
+    """
+    original_path = os.environ["PATH"]
+    with tempfile.TemporaryDirectory() as tmpdir:
+        cmd_path = pathlib.Path(tmpdir) / cmd_name
+        cmd_path.write_text(script, encoding="utf8")
+        cmd_path.chmod(0o755)
+        os.environ["PATH"] = f"{tmpdir}:{original_path}"
+        try:
+            yield
+        finally:
+            os.environ["PATH"] = original_path