calibre: tests: functional: Fix occasional failure in add book test

- When a library is added to using the FreedomBox interface and immediately
Calibre interface is loaded, the library does not immediately get listed in the
list of libraries. We will have to fresh the page to see the new library. Do
this.

Tests:

- Run functional tests for calibre on Testing distribution multiple times
without failures.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/modules/calibre/tests/test_functional.py

@@ -105,8 +105,17 @@ def _visit_library(browser, name):
 
     functional.eventually(_service_available)
 
-    functional.eventually(browser.find_by_css,
-                          args=[f'.calibre-push-button[data-lid="{name}"]'])
+    def _library_available():
+        """Refresh until the expected library is available."""
+        available = browser.find_by_css(
+            f'.calibre-push-button[data-lid="{name}"]')
+        if not available:
+            time.sleep(0.5)
+            functional.visit(browser, '/calibre')
+
+        return available
+
+    functional.eventually(_library_available)
     link = browser.find_by_css(f'.calibre-push-button[data-lid="{name}"]')
     if not link:
         raise ValueError('Library not found')