Fix tests failure on busybox systems. (#7916)

(cherry picked from commit ef92f507dd)

tests/support/server.tcl

@@ -94,7 +94,7 @@ proc kill_server config {
 
 proc is_alive config {
     set pid [dict get $config pid]
-    if {[catch {exec ps -p $pid} err]} {
+    if {[catch {exec kill -0 $pid} err]} {
         return 0
     } else {
         return 1

tests/support/util.tcl

@@ -508,7 +508,7 @@ proc populate {num prefix size} {
 
 proc get_child_pid {idx} {
     set pid [srv $idx pid]
-    if {[string match {*Darwin*} [exec uname -a]]} {
+    if {[file exists "/usr/bin/pgrep"]} {
         set fd [open "|pgrep -P $pid" "r"]
         set child_pid [string trim [lindex [split [read $fd] \n] 0]]
     } else {