Allow accessing the entry id in test files

[k33nice]

Hey, thanks for your work! I think it might be nice to have the ability to access the entry ID inside the hurl test file.
E.g.:

#1.hurl
POST /foo
{
  "must_be_unique": {{ENTRY_ID}}
}
---EOF---
#2.hurl
POST /foo
{
  "must_be_unique": {{ENTRY_ID}}
}

Thus, it could be running as hurl --test *.hurl

It seems similar to #606. But it looks more simple, cuz it already there:

hurl/packages/hurl/src/runner/hurl_file.rs

Line 99 in 802d783

for (entry_index, entry) in hurl_file

Also, sequential ID can be handier rather than random in some cases.

[fabricereix]

Hello @k33nice ,
this is a good idea.
We could have predefined variables available at runtime.
we could access the current entry index with {{self.entry.index}} or {{entry.index}}.
We initially though about them to get the current time.

Generating sequential id might also be indeed useful.
That might be enough for uniqueness in a test.