internal/worker: use testAppVersion constant in fetch_test.go
internal/worker: use testAppVersion constant in fetch_test.go

A testAppVersion constant is defined and used in place of
the "appVersionLabel" string in fetch_test.go.

Change-Id: I75cd95225c1d7337815b341d76936a3862d9a1af
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/247189
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite