cmd/objdump: speed up tests
cmd/objdump: speed up tests

Rebuild cmd/objdump once instead of twice.
Speeds up standalone 'go test cmd/objdump' on my
machine from ~1.4s to ~1s.

Updates #17751

Change-Id: I15fd79cf18c310f892bc28a9e9ca47ee010c989a
Reviewed-on: https://go-review.googlesource.com/32673
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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