BATS: Make variable persistent across all tests

I'm writing a BATS (Bash Automated Testing System) script and what I'd like is a variable to be persisted across all the tests. For example:

#!/usr/bin/env bats

# Generate random port number
port_num=$(shuf -i 2000-65000 -n 1)

@test "Test number one" {
    a = $port_num
}

@test "Test number two" {
    b = $port_num
}

When evaluated, a and b should equal each other. This doesn't work, though, because (according to the docs) the entire file is evaluated after each test run. Which means $port_num gets regenerated between tests. Is there a way/place for me to store variables that will be persisted for across all tests?