Begin Philomena Search Syntax parser snapshot tests

[MareStare]

Added the initial plumbing for Philomena Search Syntax parser tests.

The bulk of this code implements the "testing engine" for the parser. The tests are defined declaratively like this:

assert_search_syntax(%{
  compile: &Philomena.Images.Query.compile/2,
  snapshot: "#{__DIR__}/search-syntax.json",
  contexts: %{
    user: [/* ... */],
    watch: [true, false]
  },
  test_cases: [
    wildcard: [
      "*",
      "artist:*",
    ]
  ]
})

This test can be invoked either via the regular mix test to validate that the existing test snapshot is fresh. Or via philomena update-phiql-tests to update the test snapshot after a change was made to the source code. The test snapshot has this format in the simplest form:

{
  "wildcard": [
    {
      "philomena": "*",
      "opensearch": {
        "match_all": {}
      }
    }
  ]
}

There can be an additional contexts key in the test case in case if the parser output differs between contexts like anon user, regular user or admin/mod user.

Todo

Fix this comment from @liamwhite in Discord:

fields which do not exist on the document and are created by a callback

Consider using toml instead of JSON and try define the test inputs directly in the snapshot?

[MareStare]

Todo rename QueryCase to PhiqlCase

[Meow]

De-ponify the test queries (since I assume it doesn't really matter what they are), we're aiming for eventual debranding and more neutral texts in the software in the future.

[Meow]

I also think calling it just "query" instead of "phiql" is less confusing. In my mind the language's name has always been just "search syntax" or "query syntax".

[MareStare]

Marking as draft until I fix the review comments

[MareStare]

This is now ready for review

Fixed the review comments and improved the code a bit:

• Unfortunately, there is practically no TOML encoder library existent in Elixir. But anyway, TOML isn't really good at representing nested data structures. In this case Opensearch query is such a data structure
• Deponified the test data
• Renamed PhiQL to "Philomena.SearchSyntax"
• Added tests for filter_id and some more test scenarios
• Changed the user_fixtures.ex a bit during the iterations, but then I ended up not using that module because current tests don't depend on the users being in the DB. However, I left the changes I made in user_fixtures.ex since I believe they may be useful in the future

[Meow]

use |> syntax here (and the next occurence)

attrs
|> Map.put(...)
|> user_fixture()

[MareStare]

Fixed in 4f90db5. Had to rebase to fix merge conflicts