test: add extra test for issue18661
test: add extra test for issue18661

Make sure that the lack of an lvalue doesn't
cause extra side-effects.

Updates #18661
Updates #18739

Change-Id: I52eb4b4a5c6f8ff5cddd2115455f853c18112c19
Reviewed-on: https://go-review.googlesource.com/36126
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

akutz/go,akutz/go,polyverse-security/go,josharian/go,nawawi/go,linux-on-ibm-z/go,cloudflare/go,vsekhar/elastic-go,polyverse-security/go,Cofyc/go,derekmarcotte/go,codestation/go,linux-on-ibm-z/go,codestation/go,polyverse-security/go,cloudflare/go,ImJasonH/go,cloudflare/go,golang/go,akutz/go,CAFxX/go,nawawi/go,linux-on-ibm-z/go,golang/go,vsekhar/elastic-go,codestation/go,ImJasonH/go,nawawi/go,cloudflare/go,derekmarcotte/go,polyverse-security/go,ImJasonH/go,CAFxX/go,nawawi/go,josharian/go,Cofyc/go,derekmarcotte/go,vsekhar/elastic-go,CAFxX/go,nawawi/go,Cofyc/go,cloudflare/go,codestation/go,ImJasonH/go,ImJasonH/go,polyverse-security/go,akutz/go,golang/go,Cofyc/go,vsekhar/elastic-go,polyverse-security/go,linux-on-ibm-z/go,cloudflare/go,derekmarcotte/go,codestation/go,linux-on-ibm-z/go,CAFxX/go,ImJasonH/go,cloudflare/go,derekmarcotte/go,josharian/go,Cofyc/go,codestation/go,golang/go,Cofyc/go,akutz/go,linux-on-ibm-z/go,josharian/go,ImJasonH/go,derekmarcotte/go,Cofyc/go,CAFxX/go,vsekhar/elastic-go,linux-on-ibm-z/go,akutz/go,polyverse-security/go,Cofyc/go,polyverse-security/go,josharian/go,codestation/go,codestation/go,josharian/go,ImJasonH/go,nawawi/go,josharian/go,CAFxX/go,akutz/go,josharian/go,golang/go,vsekhar/elastic-go,golang/go,vsekhar/elastic-go,nawawi/go,christopher-henderson/Go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,akutz/go,linux-on-ibm-z/go,derekmarcotte/go,nawawi/go,derekmarcotte/go,vsekhar/elastic-go,golang/go