test: Fix race condition in TestPages.testFrameReload()
test: Fix race condition in TestPages.testFrameReload()

After tickling the frame to reload, wait for that to happen. Otherwise,
if that happens a bit more slowly, the subsequent `data-ready` wait is a no-op, and
`#file-content` never gets visible as the test still looks into the old
(and now destroyed) page context.

Closes #14101
martinpitt/cockpit,deryni/cockpit,martinpitt/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit,deryni/cockpit,deryni/cockpit,mvollmer/cockpit,deryni/cockpit,mvollmer/cockpit,deryni/cockpit,moraleslazaro/cockpit