Fix & re-enable flaky tests in CI

Many tests had to be disabled to render CI tests reliable.

The following tests were disabled/marked with xfail with #1041 (resolving #1039) and we should investigate whether we can "fix" something and re-enable them at some point:

• Phpactor tests on Windows
  • Finding references always fails (see #1022)
  • Other tests sometimes fail
• TestFSharpLanguageServer
  • test_find_referencing_symbols_across_files
  • test_find_referencing_symbols
  • test_hover_information
  • test_go_to_definition
• TestNixLanguageServer::test_hover_information
• TestSwiftLanguageServerBasics::test_request_references_*
• TestSerenaAgent
  • test_find_symbol_fsharp
  • test_find_symbol_rust
  • test_find_symbol_references_fsharp
  • test_find_symbol_references_typescript
• TestRLanguageServer
  • test_find_definition_across_files
  • test_find_references_across_files