Feat/inject value for tentative in simulator

[ysugimoto]

This PR enables to inject/override variables that returns tentative values written in https://github.com/ysugimoto/falco/blob/main/docs/variables.md for testing.
Sometimes the user would like to check the if branch by some fastly predefined variables so it can do testing by injecting suitable values.

Syntax

This feature enables only in testing:

sub test_override_server_region {
  // Override server.region variable value
  testing.inject_variable("server.region", "ASIA");
  testing.call_subroutine("vcl_recv");

  assert.equal(req.http.Region, "ASIA");
}

By calling testing.inject_variable with variable name and override value, the simulator looks up that value and return it if found.
Note that this function could override tentative values which are written in https://github.com/ysugimoto/falco/blob/main/docs/variables.md, typical variables could not override.