Detect adding and updating .result files

Part of #23
Related to tarantool/tarantool#5000

Author: ligurio

Diff for: checkpatch.pl

@@ -2140,6 +2140,7 @@ sub process {
 
 	my %commit_log_tags = ();
 	my $has_changelog = 0;
+	my $warned_about_test_result_file = 0;
 	my $has_doc = 0;
 	my $has_test = 0;
 	my $is_test = 0;
@@ -2823,6 +2824,14 @@ sub process {
 			      "please, use spaces instead of tabs\n" . $herevet);
 		}
 
+		if (!$warned_about_test_result_file &&
+		     $realfile =~ /^test\/.*\.result$/ &&
+		     $line =~ /^\+/) {
+			$warned_about_test_result_file = 1;
+			ERROR("TEST_RESULT_FILE",
+			      "Please avoid tests with .result files\n");
+		}
+
 # check we are in a valid C source file if not then ignore this hunk
 		next if ($realfile !~ /\.(h|c|cc)$/);
 

Diff for: doc/checkpatch.rst

@@ -774,6 +774,11 @@ Others
   **PRINTF_0XDECIMAL**
     Prefixing 0x with decimal output is defective and should be corrected.
 
+  **TEST_RESULT_FILE**
+    For regression tests, there are .result files. Tests with .result files
+    should be avoided. It is recommended to use Luatest or TAP for Tarantool
+    regression tests.
+
   **TYPO_SPELLING**
     Some words may have been misspelled.  Consider reviewing them.