test: fix flaky swim/errinj.test.lua

In one place that test sends a packet and expects that it has
arrived two lines below. Under high load it may take more time.
The patch makes the test explicitly wait for the packet arrival.

Closes #4392

test/swim/errinj.result

@@ -60,6 +60,9 @@ errinj.set("ERRINJ_SWIM_FD_ONLY", false)
 ---
 - ok
 ...
+while s1_view:incarnation().version ~= 2 do fiber.sleep(0.01) end
+---
+...
 s1_view:payload()
 ---
 - null

test/swim/errinj.test.lua

@@ -26,6 +26,7 @@ s1:self():incarnation()
 errinj.set("ERRINJ_SWIM_FD_ONLY", true)
 s1:probe_member(s2:self():uri())
 errinj.set("ERRINJ_SWIM_FD_ONLY", false)
+while s1_view:incarnation().version ~= 2 do fiber.sleep(0.01) end
 s1_view:payload()
 s1_view:incarnation()