tests: parametrize "sleep 1"

* tests/init.sh (SLEEP_A_BIT): New variable.
* tests/detach-sleeping: Use it.
* tests/detach-stopped: Likewise.
diff --git a/tests/init.sh b/tests/init.sh
index a3ff2f8..3976a1e 100644
--- a/tests/init.sh
+++ b/tests/init.sh
@@ -19,3 +19,4 @@
 
 : "${STRACE:=../strace}"
 : "${TIMEOUT_DURATION:=60}"
+: "${SLEEP_A_BIT:=sleep 1}"