go.testEnvFile should support the export key word in environment files

[sapenmsoft]

Is your feature request related to a problem? Please describe.
I want to be able to use an environment file for both inside and outside of my tests. I may have an environment file

export FOO=bar
export BAR=foo

source myfile.env
go test ./...

The variables need to be exported so they are picked up by the go subprocess. However, this file is not currently processed correctly by the go plugin.

Describe the solution you'd like
I'd like the environment files to parse both types of variable setting e.g.

export FOO=BAR
BAR=FOO

Describe alternatives you've considered
Two env files, one for go, one for bash, but this introduces too much duplication.

[gopherbot]

Change https://go.dev/cl/706135 mentions this issue: Support exported environment variables with go.testEnvFile

[h9jiang]

Thanks, I think this is reasonable change.

Please see the comment in the CL.