net/http: skip TestServerHijackGetsBackgroundByte on Plan 9
net/http: skip TestServerHijackGetsBackgroundByte on Plan 9

CL 5232 added TestServerHijackGetsBackgroundByte, which is failing
on Plan 9, because CloseWrite is not implemented on Plan 9 yet.

Updates #17906.
Updates #18657.

Change-Id: I3c2f73760b0f767f3f9ed2698c855372170e0481
Reviewed-on: https://go-review.googlesource.com/35178
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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