test: raise the meson timeout to 900s — the suite has grown past 420s with rounds 4-5

meson.build

@@ -188,5 +188,5 @@ clanker_saddle_exe = executable('clanker-saddle',
 python_test = import('python').find_installation('python3')
 test('web-stack', python_test,
     args: [files('test/web_stack_test.py'), clanker_saddle_exe],
-    timeout: 420,
+    timeout: 900,
 )