[libFuzzer] Ignore exit status of wait in merge-sigusr.test.
If process $PID has already exited, wait will give a non-zero exit
status.
@@ -27,7 +27,7 @@ RUN: %run %t/LFSIGUSR -merge=1 -merge_control_file=%t/MCF %t/C1 %t/C2 2>%t/log &
RUN: sleep 3
RUN: pkill -SIGUSR2 -P $PID
RUN: kill -SIGUSR2 $PID
-RUN: wait $PID
+RUN: wait $PID || true
RUN: cat %t/log | FileCheck %s --dump-input=fail
RUN: grep C2/g %t/MCF
RUN: grep STARTED %t/MCF
