Use tmp_path in ghostwriter tests

Before creating the temporary functions, moves the local directory to a
clean temporary directory; this should guarantee that the files to be
created don't exist, and handles cleanup automatically.

This also works when the local directory that the tests are executed
from is not writable, but the temporary directory is.

hypothesis-python/tests/ghostwriter/test_ghostwriter.py

@@ -11,6 +11,7 @@
 import ast
 import enum
 import json
+import os
 import re
 import socket
 import unittest
@@ -450,13 +451,22 @@ def test_get_imports_for_strategy(strategy, imports):
 
 
 @pytest.fixture
-def temp_script_file():
-    """Fixture to yield a Path to a temporary file in the local directory. File name will end
-    in .py and will include an importable function.
+def in_temp_path(tmp_path):
+    """Fixture to execute tests in a temporary path."""
+    old_path = Path.cwd()
+    os.chdir(tmp_path)
+    yield
+    os.chdir(old_path)
+
+
+@pytest.fixture
+def temp_script_file(in_temp_path):
+    """Fixture to create a script file in a temporary working directory.
+
+    Changes the working directory to a temporary directory, then yields an extant file
+    whose name will end in .py and which includes an importable function.
     """
     p = Path("my_temp_script.py")
-    if p.exists():
-        raise FileExistsError(f"Did not expect {p} to exist during testing")
     p.write_text(
         dedent(
             """
@@ -467,17 +477,16 @@ def say_hello():
         encoding="utf-8",
     )
     yield p
-    p.unlink()
 
 
 @pytest.fixture
-def temp_script_file_with_py_function():
-    """Fixture to yield a Path to a temporary file in the local directory. File name will end
-    in .py and will include an importable function named "py"
+def temp_script_file_with_py_function(in_temp_path):
+    """Fixture to create a python file in a temporary working directory.
+
+    Changes the working directory to a temporary directory, then yields an extant file
+    whose name will end in .py and which includes an importable function named "py".
     """
     p = Path("my_temp_script_with_py_function.py")
-    if p.exists():
-        raise FileExistsError(f"Did not expect {p} to exist during testing")
     p.write_text(
         dedent(
             """
@@ -488,7 +497,6 @@ def py():
         encoding="utf-8",
     )
     yield p
-    p.unlink()
 
 
 def test_obj_name(temp_script_file, temp_script_file_with_py_function):