pgsql: Add a new helper function wait_for_file() to Utils.pm.

Add a new helper function wait_for_file() to Utils.pm.

wait_for_file() waits for the contents of a specified file, starting at an
optional offset, to match a given regular expression. If no offset is
provided, the entire file is checked. The function times out after
$PostgreSQL::Test::Utils::timeout_default seconds. It returns the total
file length on success.

The existing wait_for_log() function contains almost identical logic, but
is limited to reading the cluster's log file. This commit also refactors
wait_for_log() to call wait_for_file() instead, avoiding code duplication.

This helper will be used by upcoming changes.

Suggested-by: Mircea Cadariu <cadariu(dot)mircea(at)gmail(dot)com>
Author: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Mircea Cadariu <cadariu(dot)mircea(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHGQGwFeTymZQ7RLvMU6WuDGar8bUQCazg=VOfA-9GeBkg-FzA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0b10969db610c607227b23710d556b595d7eb34a

Modified Files
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src/test/perl/PostgreSQL/Test/Cluster.pm | 20 ++--------------
src/test/perl/PostgreSQL/Test/Utils.pm | 39 ++++++++++++++++++++++++++++++++
2 files changed, 41 insertions(+), 18 deletions(-)