test: Add GHA job leak-test

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[erikmd]

FTR, excerpt of the log of this run of the leak-test job:

→ Define and export env vars

***
***

→ Run coq-community/docker-coq-action@v1 with debian:10

+ (script @ line 4) $ : A. This should print ok twice, and okok twice
+ (script @ line 5) $ echo ok
ok
+ (script @ line 6) $ echo okok
okok

+ (script @ line 8) $ : B. This should print '***' thrice
+ (script @ line 9) $ echo ***
***
+ (script @ line 10) $ echo ***
***
+ (script @ line 11) $ echo ***
***

C. This should print ok only once
ok

triggerred by this job.