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
```

test/mod/test_testutil_mock_command.py

@@ -0,0 +1,29 @@
+#
+# Tests for the 'osbuild.util.testutil.mock_command' module.
+#
+import os
+import subprocess
+import textwrap
+
+from osbuild.testutil import mock_command
+
+
+def test_mock_command_integration():
+    output = subprocess.check_output(["echo", "hello"])
+    assert output == b"hello\n"
+    fake_echo = textwrap.dedent("""\
+    #!/bin/sh
+    echo i-am-not-echo
+    """)
+    with mock_command("echo", fake_echo):
+        output = subprocess.check_output(["echo", "hello"])
+        assert output == b"i-am-not-echo\n"
+    output = subprocess.check_output(["echo", "hello"])
+    assert output == b"hello\n"
+
+
+def test_mock_command_environ_is_modified_and_restored():
+    orig_path = os.environ["PATH"]
+    with mock_command("something", "#!/bin/sh\ntrue\n"):
+        assert os.environ["PATH"] != orig_path
+    assert os.environ["PATH"] == orig_path