(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/ui/BillingPage.ts

@@ -557,7 +557,7 @@ export class BillingPage extends Disposable {
             testId('company-address-country')
           )
         ) : null,
-      ] : 'Fetching address...'
+      ] : css.billingHintText('Fetching address...')
     );
   }
 
@@ -568,7 +568,7 @@ export class BillingPage extends Disposable {
         css.summaryHeader(
           css.billingBoldText(`Payment Card`),
         ),
-        'Fetching card preview...'
+        css.billingHintText('Fetching card preview...')
       );
     } else if (card) {
       // There is a card attached to the account.