Adding windows build constraints to code to allow tests and benchmarks
to be run on Linux.

Added doc.go to modules (with doc string, where appropriate) to prevent
compiler/linter errors about broken imports.
In some cases (ie, winapi and wclayer), the package already had an
OS-agnostic file of the same name, along with a doc string. A doc.go
file was added to preempt situations where windows-specific code is
added to that file in the future.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

Renaming files in `test\cri-containerd` to end with `_test.go` so they
can include variables and functions defined in other `_test.go`. For
example, `gmsa.go` imports `gmsaAccount`, which is defined in
`main_test.go`.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>