fix(pdf): threadsafe for pypdfium2 backend (#2527)

* add threadsafe test

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* test backend

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* test threaded pipeline

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* add test_pypdfium_threaded_pipeline

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* add more threadsafe blocks

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* fix threadsafe in pypdfium backend

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* remove unneccessary tests

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* restore clean test

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

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Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>