functional-tests: snapshot: Skip if filesystem doesn't support snapshots

Tested that on the Btrfs filesystem snapshot functional tests pass and
on the ext4 filesystem tests are skipped.

Signed-off-by: Veiko Aasa <veiko17@disroot.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/snapshot/templates/snapshot_not_supported.html

@@ -7,7 +7,7 @@
 {% load i18n %}
 
 {% block configuration %}
-  <div class="alert alert-danger">
+  <div id="snapshot-not-supported" class="alert alert-danger">
     {% blocktrans trimmed with fs_types_supported|join:', ' as types_supported %}
       You have a filesystem of type <strong>{{ fs_type }}</strong>. Snapshots
       are currently only available on <strong>{{ types_supported }}</strong>

plinth/modules/snapshot/tests/snapshot.feature

@@ -6,7 +6,8 @@ Feature: Storage Snapshots
 
 Background:
   Given I'm a logged in user
-  Given the snapshot application is installed
+  And the snapshot application is installed
+  And the filesystem supports snapshots
 
 Scenario: Create a snapshot
   Given the list of snapshots is empty

plinth/modules/snapshot/tests/test_functional.py

@@ -3,6 +3,7 @@
 Functional, browser based tests for snapshot app.
 """
 
+import pytest
 from pytest_bdd import given, parsers, scenarios, then, when
 
 from plinth.tests import functional
@@ -10,6 +11,13 @@ from plinth.tests import functional
 scenarios('snapshot.feature')
 
 
+@given('the filesystem supports snapshots')
+def is_snapshots_supported(session_browser):
+    if not _is_snapshot_supported(session_browser):
+        pytest.skip('Filesystem doesn\'t support snapshots')
+    assert True
+
+
 @given('the list of snapshots is empty')
 def empty_snapshots_list(session_browser):
     _delete_all(session_browser)
@@ -99,6 +107,12 @@ def _get_count(browser):
     return len(browser.find_by_xpath('//tr')) - 1
 
 
+def _is_snapshot_supported(browser):
+    """Return whether the filesystem supports snapshots."""
+    functional.nav_to_module(browser, 'snapshot')
+    return not bool(browser.find_by_id('snapshot-not-supported'))
+
+
 def _set_configuration(browser, free_space, timeline_enabled, software_enabled,
                        hourly, daily, weekly, monthly, yearly):
     """Set the configuration for snapshots."""