go/build: update TestImportDirNotExist to accept more detailed error strings
In CL 203820, we switched go/build to use the caller's working
directory for the main module (rather than srcDir), so that go/build
resolution now respects the requirements and replacements of the main
module. When the passed-in srcDir is empty, as of that CL we use "go
list" instead of falling back to in-process (GOPATH-mode) path lookup.
Unfortunately, that broke go/build.TestImportDirNotExist when
GO111MODULE=on: the test was looking for the specific error message
produced by the in-process lookup.
This change relaxes the test to accept the error message produced by
"go list" when srcDir is empty.