BraveRewardsBrowserTest.* tests frequently time out

Brave Rewards tests are frequently timing out which causes each PR to take longer and also takes longer for people checking tests locally on their tasks.

As an example:

BraveRewardsBrowserTest.ProcessPendingContributions (TIMED OUT)

But there are others too (8) that frequently timeout.

cc @NejcZdovc

Here are some ideas for cleaning up tests:

• Add a method to enable Rewards without using the Rewards Page UI, and switch tests to it where possible. (#7002)
• Replace use of sleep/polling with a MutationObserver that will watch for the page to be modified. (https://github.com/brave/brave-browser/issues/7060)
• Switch tests to use TipViaCode where possible. (https://github.com/brave/brave-browser/issues/7184)
• Add a method to claim an initial grant without going through the Rewards Page or Rewards Panel UI, and switch tests to using it where possible. (https://github.com/brave/brave-browser/issues/9129)
• Add a method to open the Site Banner without going through the Rewards Panel, and switch tests to using it where possible.
• Move utility functions into a separate file.
• Split Rewards Service tests into multiple files based on test category. (https://github.com/brave/brave-browser/issues/9204)

[kjozwiak]

Just an update, still seeing the following failure from time to time:

03:29:38  [  FAILED  ] BraveRewardsBrowserTest.ProcessPendingContributions, where TypeParam =  and GetParam() =  (37759 ms)
03:29:38  [539/540] BraveRewardsBrowserTest.ProcessPendingContributions (38025 ms)
03:29:44  [540/540] BraveRewardsBrowserTest.SplitProcessorAutoContribution (5241 ms)
03:29:44  1 test failed:
03:29:44      BraveRewardsBrowserTest.ProcessPendingContributions (../../brave/components/brave_rewards/browser/rewards_service_browsertest.cc:2186)

[NejcZdovc]

closing as nothing new is planned for this and browser tests are stable now