Fix the flakiness ee/spec/features/search/zoekt/search_spec.rb

What does this MR do and why?

I suspect that due to parallel testing, the two different examples of regex search and exact code search somehow mess with the mode selection as the mode gets saved in the cache. When we click on the regex toggle, we might be doing the opposite of what we want. To fix this, I have merged the two examples of regex search and exact code search into one, so they always run in a given order. I have moved out the permission test to a different example to make it clearer.

Also used before_all and after_all to prepare and destroy the test data only once for all examples.

Removed the FF disabled stubbing of zoekt_multimatch_frontend and zoekt_cross_namespace_search.

I found a bug with the zoekt search by proxy so added the FF disabled stubbing zoekt_search_proxy.

The run time of specs is still more than the recommended. We will change the test setup of zoekt indexer with this issue #525092. We can see the timing after this issue gets fixed. This MR should fix the flakiness. For now, I will make it allowed_to_be_slow

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Related to #490987 (closed)