pg_bsd_indent: Collect test diffs in test.diffs instead of tests.diff

That way CI knows to pick up the file if the test fails.

Discussion: https://postgr.es/m/20230711233307.hu4wetabjm5f7ver@awork3.anarazel.de
Backpatch: 16-, where the test was added

src/tools/pg_bsd_indent/t/001_pg_bsd_indent.pl

@@ -24,7 +24,7 @@ program_version_ok('pg_bsd_indent');
 
 # Run pg_bsd_indent on pre-fab test cases.
 # Any diffs in the generated files will be accumulated here.
-my $diff_file = "tests.diff";
+my $diffs_file = "test.diffs";
 
 # Copy support files to current dir, so *.pro files don't need to know path.
 while (my $file = glob("$src_dir/tests/*.list"))
@@ -44,9 +44,9 @@ while (my $test_src = glob("$src_dir/tests/*.0"))
 			"$test.out", "-P$src_dir/tests/$test.pro"
 		],
 		"pg_bsd_indent succeeds on $test");
-	# check result matches, adding any diff to $diff_file
+	# check result matches, adding any diff to $diffs_file
 	my $result = run_log([ 'diff', '-upd', "$test_src.stdout", "$test.out" ],
-		'>>', $diff_file);
+		'>>', $diffs_file);
 	ok($result, "pg_bsd_indent output matches for $test");
 }