Fix leftover processes/hangs in tests

One of such cases is 02479_race_condition_between_insert_and_droppin_mv
[1], yes it can be fixed (by using fixed number of iterations, or with
some bash trickery), but it is better to fix them completelly,
eventually such tests will be submitted and pass review anyway.

By allocating process group for each test we can kill all the processes
in this process group, and this what this patch does.

This will also fix some test hangs (like in [1]) as well as some
possible issues in stress tests.

  [1]: https://s3.amazonaws.com/clickhouse-test-reports/0/e2c1230b00386c4d0096a245396ab3be7ce60950/stateless_tests__release__analyzer_/run.log

Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>
(cherry picked from commit 72fa58e192)

tests/clickhouse-test

@@ -1031,7 +1031,7 @@ class TestCase:
         if proc:
             if proc.returncode is None:
                 try:
-                    proc.kill()
+                    os.killpg(os.getpgid(proc.pid), signal.SIGKILL)
                 except OSError as e:
                     if e.errno != ESRCH:
                         raise
@@ -1307,7 +1307,7 @@ class TestCase:
 
         command = pattern.format(**params)
 
-        proc = Popen(command, shell=True, env=os.environ)
+        proc = Popen(command, shell=True, env=os.environ, start_new_session=True)
 
         while (
             datetime.now() - start_time

tests/queries/shell_config.sh

@@ -185,3 +185,11 @@ function query_with_retry
     done
     echo "Query '$query' failed with '$result'"
 }
+
+# Add --foreground to avoid running setpgid() in timeout, otherwise
+# clickhouse-test will not kill those processes in case of timeout
+function timeout()
+{
+    command timeout --foreground "$@"
+}
+export -f timeout