Detect adding and updating .result files

Part of #23
Related to tarantool/tarantool#5000

checkpatch.pl

@@ -2140,6 +2140,8 @@ sub process {
 
 	my %commit_log_tags = ();
 	my $has_changelog = 0;
+	my $has_test_result_file = 0;
+	my $warned_about_result_file = 0;
 	my $has_doc = 0;
 	my $has_test = 0;
 	my $is_test = 0;
@@ -2731,6 +2733,9 @@ sub process {
 		if ($realfile =~ /^(?:static-build\/)?test\/.*\//) {
 			$has_test = 1;
 		}
+		if ($realfile =~ /^test\/.*\.result$/) {
+			$has_test_result_file = 1;
+		}
 
 # check for repeated words separated by a single space
 # avoid false positive from list command eg, '-rw-r--r-- 1 root root'
@@ -2823,6 +2828,14 @@ sub process {
 			      "please, use spaces instead of tabs\n" . $herevet);
 		}
 
+		if (!$warned_about_result_file &&
+		     $has_test_result_file &&
+		     $line =~ /^\+/) {
+			$warned_about_result_file = 1;
+			ERROR("TEST_DIFF_FILE",
+			      "Please avoid tests with .result files\n");
+		}
+
 # check we are in a valid C source file if not then ignore this hunk
 		next if ($realfile !~ /\.(h|c|cc)$/);
 

doc/checkpatch.rst

@@ -742,6 +742,10 @@ Others
     For DOS-formatted patches, there are extra ^M symbols at the end of
     the line.  These should be removed.
 
+  **TEST_DIFF_FILE**
+    For regression tests, there are .result files. Tests with .result files
+    should be avoided.
+
   **MEMSET**
     The memset use appears to be incorrect.  This may be caused due to
     badly ordered parameters.  Please recheck the usage.