Tests: Some fixes for macOS

1) cur_test: when restart_server, "no such variable" error occurs
  ./runtest --single integration/rdb
  test {client freed during loading}
      SET ::cur_test
      restart_server
        kill_server
          test "Check for memory leaks (pid $pid)"
          SET ::cur_test
          UNSET ::cur_test
      UNSET ::cur_test // This global variable has been unset.

2) `ps --ppid` not available on macOS platform, can be replaced with
`pgrep -P pid`.

(cherry picked from commit e90385e2232d41fd7c40dc239279f9837e7bdf57)

tests/support/test.tcl

@@ -190,5 +190,4 @@ proc test {name code {okpattern undefined} {options undefined}} {
             send_data_packet $::test_server_fd err "Detected a memory leak in test '$name': $output"
         }
     }
-    unset ::cur_test
 }

tests/support/util.tcl

@@ -494,3 +494,28 @@ proc start_bg_complex_data {host port db ops} {
 proc stop_bg_complex_data {handle} {
     catch {exec /bin/kill -9 $handle}
 }
+
+proc populate {num prefix size} {
+    set rd [redis_deferring_client]
+    for {set j 0} {$j < $num} {incr j} {
+        $rd set $prefix$j [string repeat A $size]
+    }
+    for {set j 0} {$j < $num} {incr j} {
+        $rd read
+    }
+    $rd close
+}
+
+proc get_child_pid {idx} {
+    set pid [srv $idx pid]
+    if {[string match {*Darwin*} [exec uname -a]]} {
+        set fd [open "|pgrep -P $pid" "r"]
+        set child_pid [string trim [lindex [split [read $fd] \n] 0]]
+    } else {
+        set fd [open "|ps --ppid $pid -o pid" "r"]
+        set child_pid [string trim [lindex [split [read $fd] \n] 1]]
+    }
+    close $fd
+
+    return $child_pid
+}

tests/unit/oom-score-adj.tcl

@@ -14,15 +14,6 @@ if {$system_name eq {linux}} {
             return $val
         }
 
-        proc get_child_pid {} {
-            set pid [srv 0 pid]
-            set fd [open "|ps --ppid $pid -o pid" "r"]
-            set child_pid [string trim [lindex [split [read $fd] \n] 1]]
-            close $fd
-
-            return $child_pid
-        }
-
         test {CONFIG SET oom-score-adj works as expected} {
             set base [get_oom_score_adj]
 
@@ -47,7 +38,7 @@ if {$system_name eq {linux}} {
             r config set rdb-key-save-delay 100000
             r bgsave
 
-            set child_pid [get_child_pid]
+            set child_pid [get_child_pid 0]
             assert {[get_oom_score_adj $child_pid] == [expr $base + 30]}
         }