Allow LSP test_file to autofill didOpen params from previous splits

[DavidLoftus]

Currently file tests for LSP must provide carbon source code as an escaped string within notify params, i.e.

[[@LSP-NOTIFY:textDocument/didOpen:
  "textDocument": {
    "uri": "file:/class.carbon",
    "languageId": "carbon",
    "text": "class A {\n  fn F();\n  fn G() {}\n}\n"
  }
]]

This works fine for simple, single line files but gets annoying when working with more complicated files which are necessary when testing more complicated features e.g. goto-definition

--- class.carbon
class A {
  fn F();
  fn G() {}
}
--- STDIN
[[@LSP-NOTIFY:textDocument/didOpen:
  "textDocument": {
    "uri": "file:/class.carbon",
    "languageId": "carbon",
    "text": "AUTOFILL"
  }
]]

This PR extends file_test_base to be able to parse the notify/call params and inject files from the test_file's splits into the JSON input. I purposely avoid using the clangd types and manually parse the llvm::json::Value here to avoid introducing a depdendency on clangd to the generic file_test_base, but happy to change if we think that is fine. Also happy to accept other suggestions on alternative methods to achieve same result.