Double free error when requiring vips in Busted tests

[scossu]

When running Busted tests, if vips is required by more than one test file, the program crashes during garbage collection.

Arch Linux, Lua 5.4.8, lua-vips 1.1-12, busted 2.2.0-1.

Minimally reproducible example:

1. Create two identical files, test1.lua and test2.lua, with the following content:

local vips = require "vips"
describe("Test", function() end)

2. Run busted test1.lua test2.lua
3. Lua crashes during garbage collection with the message: free(): double free detected in tcache 2.

I am unfamiliar with the way busted parses and runs the files, so I wasn't able to determine what exactly is causing the crash.

[scossu]

This may or may not be related to #101 given the same environment and the garbage collector involved.