os/user: align test file name and build tags

TestNegativeUid tests code in cgo_lookup_unix.go, so let's use
the very same go:build line for the test file, and rename
the test file accordingly.

Change-Id: Iee3dbc25aeb8a7659c734d97dde3e9c670e80fae
Reviewed-on: https://go-review.googlesource.com/c/go/+/702976
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Mark Freeman <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: kolyshkin

src/os/user/cgo_unix_test.go renamed to src/os/user/cgo_lookup_unix_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (darwin || dragonfly || freebsd || (!android && linux) || netbsd || openbsd || solaris) && cgo && !osusergo
+//go:build (cgo || darwin) && !osusergo && unix && !android
 
 package user