Refactor TAP test code for file comparisons into new routine in Utils.pm

Refactor TAP test code for file comparisons into new routine in Utils.pm

This unifies the output used should any differences be found in the
files provided, information that 027_stream_regress did not show on
failures.  TAP tests of pg_combinebackup and pg_upgrade now rely on the
refactored routine, reducing the dependency to the diff command.  The
callers of this routine can optionally specify a custom line-comparison
function.

There are a couple of tests that still use directly a diff command:
001_pg_bsd_indent, 017_shm and test_json_parser's 003.  These rely on
different properties and are left out for now.

Extracted from a larger patch by the same author.

Author: Ashutosh Bapat
Discussion: https://postgr.es/m/Z6RQS-tMzGYjlA-H@paquier.xyz