Fix TestStream_AlertSNMPTrap

Test is flaky
```
--- FAIL: TestStream_AlertSNMPTrap (0.21s)
streamer_test.go:10486: digraph TestStream_Alert {
stream0 -> from1;
from1 -> window2;
window2 -> count3;
count3 -> alert4;
}
streamer_test.go:8487: unequal lists ignoring order:
got
([]interface {}) (len=1 cap=1) {
(snmptraptest.Trap) {
Pdu: (snmptraptest.Pdu) {
IsNil: (bool) false,
Type: (snmpgo.PduType) SNMPTrapV2,
ErrorStatus: (snmpgo.ErrorStatus) NoError,
ErrorIndex: (int) 0,
VarBinds: (snmptraptest.VarBinds) (len=4 cap=4) {
(snmptraptest.VarBind) {
Oid: (string) (len=21) "1.3.6.1.6.3.1.1.4.1.0",
Value: (string) (len=5) "1.1.2",
Type: (string) (len=3) "Oid"
},
(snmptraptest.VarBind) {
Oid: (string) (len=7) "1.1.2.3",
Value: (string) (len=2) "10",
Type: (string) (len=7) "Integer"
},
(snmptraptest.VarBind) {
Oid: (string) (len=7) "1.1.2.3",
Value: (string) "",
Type: (string) (len=4) "Null"
},
(snmptraptest.VarBind) {
Oid: (string) (len=7) "1.1.2.3",
Value: (string) (len=5) "20000",
Type: (string) (len=9) "TimeTicks"
}
}
},
Error: (error)
}
}

exp
([]interface {}) (len=2 cap=2) {
(snmptraptest.Trap) {
Pdu: (snmptraptest.Pdu) {
IsNil: (bool) false,
Type: (snmpgo.PduType) SNMPTrapV2,
ErrorStatus: (snmpgo.ErrorStatus) NoError,
ErrorIndex: (int) 0,
VarBinds: (snmptraptest.VarBinds) (len=4 cap=4) {
(snmptraptest.VarBind) {
Oid: (string) (len=21) "1.3.6.1.6.3.1.1.4.1.0",
Value: (string) (len=5) "1.1.1",
Type: (string) (len=3) "Oid"
},
(snmptraptest.VarBind) {
Oid: (string) (len=7) "1.1.1.2",
Value: (string) (len=1) "1",
Type: (string) (len=9) "Counter64"
},
(snmptraptest.VarBind) {
Oid: (string) (len=7) "1.1.1.2",
Value: (string) (len=10) "SNMP ALERT",
Type: (string) (len=11) "OctetString"
},
(snmptraptest.VarBind) {
Oid: (string) (len=7) "1.1.1.2",
Value: (string) (len=33) "kapacitor/cpu/serverA is CRITICAL",
Type: (string) (len=11) "OctetString"
}
}
},
Error: (error)
},
(snmptraptest.Trap) {
Pdu: (snmptraptest.Pdu) {
IsNil: (bool) false,
Type: (snmpgo.PduType) SNMPTrapV2,
ErrorStatus: (snmpgo.ErrorStatus) NoError,
ErrorIndex: (int) 0,
VarBinds: (snmptraptest.VarBinds) (len=4 cap=4) {
(snmptraptest.VarBind) {
Oid: (string) (len=21) "1.3.6.1.6.3.1.1.4.1.0",
Value: (string) (len=5) "1.1.2",
Type: (string) (len=3) "Oid"
},
(snmptraptest.VarBind) {
Oid: (string) (len=7) "1.1.2.3",
Value: (string) (len=2) "10",
Type: (string) (len=7) "Integer"
},
(snmptraptest.VarBind) {
Oid: (string) (len=7) "1.1.2.3",
Value: (string) "",
Type: (string) (len=4) "Null"
},
(snmptraptest.VarBind) {
Oid: (string) (len=7) "1.1.2.3",
Value: (string) (len=5) "20000",
Type: (string) (len=9) "TimeTicks"
}
}
},
Error: (error)
}
}

```

Start with TestStream_AlertSNMPTrap in streamer_test.go, especially the failure locations at lines 10486 and 8487, and run the test repeatedly to reproduce the flaky result. Trace why the expected two SNMP traps are sometimes reduced to one; done means the test reliably receives and validates both expected traps.

Written by the indexing model from the issue text.