[test][zxio] Relocate unit tests to exist alongside source

Change-Id: Iec0b5b83dbcabb41d64f6c13682ac8bb3ddf68e3

zircon/system/utest/zxio/BUILD.gn → zircon/system/ulib/zxio/test/BUILD.gn

@@ -2,6 +2,13 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+group("test") {
+  testonly = true
+  deps = [
+    ":zxio",
+  ]
+}
+
 test("zxio") {
   sources = [
     "null-test.cpp",

zircon/system/utest/BUILD.gn

@@ -86,6 +86,7 @@ if (current_cpu != "") {
       "$zx/system/ulib/trace-vthread:tests",
       "$zx/system/ulib/zbi-bootfs/test",
       "$zx/system/ulib/zx/test",
+      "$zx/system/ulib/zxio/test",
       "$zx/system/ulib/zxtest/test",
       "abigen",
       "async-testutils",
@@ -193,7 +194,6 @@ if (current_cpu != "") {
       "vmo",
       "zbi",
       "zxcrypt",
-      "zxio",
 
       # disabled for now:
       #"bad-kernel-access",