test: pin interpolation of an account var whose origin is another account

[ascandone]

Adds a spec fixture for interpolating an account variable whose origin is another account:

vars {
  account $a = @src
}
send [USD/2 10] (
  source = @world
  destination = @dest:$a
)

It must post to dest:src. The interpreter already does, so this is green on main.

The fixture corpus also runs through the compiler+VM in #199, where it is red today: the VM posts to dest:world. The fix lands in #199, which has the compiler.

[NumaryBot]

The required automated review completed with no remaining findings.

[ascandone]

Superseded by #209, which merged with the same fixture (e2e/account-interpolation-account-alias).