(core) Fix several flaky tests.

Summary:
- Fix flaky SamplesWS test, which was occasionally clicking a disabled button
- Increase timeout on backupSqliteDatabase test, which sometimes times out
- Fix a little flakiness in ExportSection test.
- Fix flaky Billing test by adjusting behavior slightly.

In Billing, when re-fetching subscription (e.g. on navigating back), we now
unset it while the fetch is pending, so that billing pages show a spinner or
"Fetching..." messages. This also gives tests something to wait for.

Also adjusts Billing styles on the "Fetching..." messages to make them look
different from regular text.

Test Plan: Mainly making exising tests more robust. Billing changes exercised by existing tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2920

app/client/models/BillingModel.ts

@@ -239,6 +239,9 @@ export class BillingModelImpl extends Disposable implements BillingModel {
   private async _fetchSubscription(forceReload: boolean = false): Promise<void> {
     if (forceReload || this.subscription.get() === undefined) {
       try {
+        // Unset while fetching for forceReload, so that the user (and tests) can tell that a
+        // fetch is pending.
+        this.subscription.set(undefined);
         const sub = await this._billingAPI.getSubscription();
         bundleChanges(() => {
           this.plans.set(sub.plans);