Add some initial Input key/action unit tests

This is just getting a feel for how I'll go about testing these. The main
focus here won't be on the bindings themselves -- they're not really
interesting and I feel could change over time anyway as people's tastes
settle down. What I want to test here are the actions that get bound.

This is just an initial small set of what's going to be a much bigger
collection of Input action tests.

tests/input/test_input_key_actions.py

@@ -0,0 +1,72 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Input
+
+
+class InputTester(App[None]):
+    """Input widget testing app."""
+
+    def compose(self) -> ComposeResult:
+        for value, input_id in (
+            ("", "empty"),
+            ("Shiny", "single-word"),
+            ("Curse your sudden but inevitable betrayal", "multi-no-punctuation"),
+            (
+                "We have done the impossible, and that makes us mighty.",
+                "multi-punctuation",
+            ),
+            ("Long as she does it quiet-like", "multi-and-hyphenated"),
+        ):
+            yield Input(value, id=input_id)
+
+
+async def test_input_home() -> None:
+    """Going home should always land at position zero."""
+    async with InputTester().run_test() as pilot:
+        for input in pilot.app.query(Input):
+            input.action_home()
+            assert input.cursor_position == 0
+
+
+async def test_input_end() -> None:
+    """Going end should always land at the last position."""
+    async with InputTester().run_test() as pilot:
+        for input in pilot.app.query(Input):
+            input.action_end()
+            assert input.cursor_position == len(input.value)
+
+
+async def test_input_right_from_home() -> None:
+    """Going right should always land at the next position, if there is one."""
+    async with InputTester().run_test() as pilot:
+        for input in pilot.app.query(Input):
+            input.action_cursor_right()
+            assert input.cursor_position == (1 if input.value else 0)
+
+
+async def test_input_right_from_end() -> None:
+    """Going right should always stay put if doing so from the end."""
+    async with InputTester().run_test() as pilot:
+        for input in pilot.app.query(Input):
+            input.action_end()
+            input.action_cursor_right()
+            assert input.cursor_position == len(input.value)
+
+
+async def test_input_left_from_home() -> None:
+    """Going left from home should stay put."""
+    async with InputTester().run_test() as pilot:
+        for input in pilot.app.query(Input):
+            input.action_cursor_left()
+            assert input.cursor_position == 0
+
+
+async def test_input_left_from_end() -> None:
+    """Going left from the end should go back one place, where possible."""
+    async with InputTester().run_test() as pilot:
+        for input in pilot.app.query(Input):
+            input.action_end()
+            input.action_cursor_left()
+            assert input.cursor_position == (len(input.value) - 1 if input.value else 0)
+
+
+# TODO: more tests.