Add parent file export smoke tests.

tests/playwright/playwright.py

@@ -5,12 +5,16 @@
 
 - the db must be reset to the baseline with demo stories
 - site must be running (currently hardcoded to port 5000)
+- start code gen in another window with `python codegen`,
+  then go to http://localhost:5000/ in the new window
+- click through etc etc, then stop the code gen, copy-paste
+  code here, fix as needed, _then_ shut down
 
 More notes:
 
 This is _just a smoke test_, it doesn't do any assertions.
 The actions were _recorded_ using playwright's supertastic
-code generation.  https://playwright.dev/docs/codegen
+code generation.  https://playwright.dev/python/docs/codegen
 
 Then I added some tweaks:
 
@@ -111,6 +115,14 @@ def _print(s):
     page.get_by_role("link", name="Sentences").click()
     page.get_by_role("link", name="Back to list").click()
 
+    _print("Export parent term mapping files.")
+    page.locator("#menu_terms").hover()
+    page.get_by_role("link", name="Parent Term mapping").click()
+    with page.expect_download() as _:
+        page.get_by_role("link", name="Tutorial", exact=True).click()
+    with page.expect_download() as _:
+        page.get_by_role("link", name="English").click()
+
     # Edit language.
     _print("Edit language.")
     page.locator("#menu_settings").hover()