os: add TestReadAtOffset
os: add TestReadAtOffset

In the Plan 9 kernel, there used to be a bug in the implementation of
the pread syscall, where the channel offset was erroneously updated after
calling pread on a file.

This test verifies that ReadAt is behaving as expected.

Fixes #14534.

Change-Id: Ifc9fd40a1f94879ee7eb09b2ffc369aa2bec2926
Reviewed-on: https://go-review.googlesource.com/22244
Run-TryBot: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

josharian/go,vsekhar/elastic-go,4ad/go,polyverse-security/go,theif519/go,vsekhar/elastic-go,ImJasonH/go,cloudflare/go,theif519/go,codestation/go,theif519/go,4ad/go,CAFxX/go,momchil-velikov/go,cloudflare/go,CAFxX/go,theif519/go,goodlang/good,derekmarcotte/go,suncycheng/go,akutz/go,derekmarcotte/go,josharian/go,josharian/go,vsekhar/elastic-go,cloudflare/go,akutz/go,golang/go,polyverse-security/go,4ad/go,akutz/go,ganboing/go-esx,derekmarcotte/go,momchil-velikov/go,ImJasonH/go,cloudflare/go,Cofyc/go,cloudflare/go,vsekhar/elastic-go,suncycheng/go,josharian/go,golang/go,nawawi/go,Cofyc/go,polyverse-security/go,linux-on-ibm-z/go,ImJasonH/go,codestation/go,vsekhar/elastic-go,cloudflare/go,ImJasonH/go,ganboing/go-esx,ganboing/go-esx,ganboing/go-esx,akutz/go,momchil-velikov/go,codestation/go,momchil-velikov/go,goodlang/good,nawawi/go,theif519/go,ganboing/go-esx,momchil-velikov/go,linux-on-ibm-z/go,golang/go,nawawi/go,CAFxX/go,goodlang/good,polyverse-security/go,vsekhar/elastic-go,derekmarcotte/go,linux-on-ibm-z/go,polyverse-security/go,suncycheng/go,linux-on-ibm-z/go,Cofyc/go,CAFxX/go,nawawi/go,golang/go,4ad/go,ganboing/go-esx,CAFxX/go,josharian/go,polyverse-security/go,akutz/go,linux-on-ibm-z/go,CAFxX/go,ImJasonH/go,josharian/go,CAFxX/go,golang/go,suncycheng/go,codestation/go,akutz/go,goodlang/good,Cofyc/go,derekmarcotte/go,josharian/go,derekmarcotte/go,nawawi/go,suncycheng/go,codestation/go,suncycheng/go,golang/go,momchil-velikov/go,cloudflare/go,cloudflare/go,suncycheng/go,polyverse-security/go,4ad/go,derekmarcotte/go,codestation/go,CAFxX/go,polyverse-security/go,akutz/go,codestation/go,4ad/go,derekmarcotte/go,josharian/go,4ad/go,goodlang/good,theif519/go,ganboing/go-esx,nawawi/go,suncycheng/go,linux-on-ibm-z/go,goodlang/good,Cofyc/go,momchil-velikov/go,ImJasonH/go,Cofyc/go,goodlang/good,codestation/go,golang/go,Cofyc/go,momchil-velikov/go,christopher-henderson/Go,linux-on-ibm-z/go,Cofyc/go,golang/go,theif519/go,theif519/go,ImJasonH/go,4ad/go,linux-on-ibm-z/go,akutz/go,vsekhar/elastic-go,ganboing/go-esx,nawawi/go,vsekhar/elastic-go,goodlang/good,nawawi/go,ImJasonH/go