Because of that it isn’t possible to run tests on PostgreSQL 15+.
Instead of relying on pg_regress the PR replaces all occurrences of @abs_srcdir@ to <root_dir>/test using sed.
It also moves input files from input and output to sql and expected directories respectively to not conflict with pg_regress (it looks for input files in case if there are such directories).
PostgreSQL 15 dropped support of .source files in tests:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=d1029bb5a26cb84b116b0dee4dde312291359f2a https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=dc9c3b0ff21465fa89d71eecf5e6cc956d647eca
Because of that it isn’t possible to run tests on PostgreSQL 15+.
@abs_srcdir@to<root_dir>/testusingsed.inputandoutputtosqlandexpecteddirectories respectively to not conflict with pg_regress (it looks for input files in case if there are such directories).