build: phase 2 wave C — test fixtures build as packages

All thirteen /test/system/services fixtures convert on the pci-bus
template (thread-test threaded; crash-test/device-list/pci-cap-test/
iommu-fault-test carrying their protocol and PCI extras). With the last
addUserBinary caller gone, the root build's wrapper functions and its
default-imports plumbing are deleted — every user binary now reaches
the shared recipe only through its own package. Production boot-image
file list unchanged; a -Dtest-case build bundles the fixtures exactly
as before.

build.zig.zon

@@ -58,6 +58,19 @@
         .@"usb-hid" = .{ .path = "system/drivers/usb-hid" },
         .@"usb-storage" = .{ .path = "system/drivers/usb-storage" },
         .@"virtio-gpu" = .{ .path = "system/drivers/virtio-gpu" },
+        .@"vfs-test" = .{ .path = "test/system/services/vfs-test" },
+        .@"fat-test" = .{ .path = "test/system/services/fat-test" },
+        .@"shared-memory-server" = .{ .path = "test/system/services/shared-memory-server" },
+        .@"shared-memory-client" = .{ .path = "test/system/services/shared-memory-client" },
+        .@"crash-test" = .{ .path = "test/system/services/crash-test" },
+        .@"device-list" = .{ .path = "test/system/services/device-list" },
+        .@"pci-cap-test" = .{ .path = "test/system/services/pci-cap-test" },
+        .@"iommu-fault-test" = .{ .path = "test/system/services/iommu-fault-test" },
+        .@"input-source" = .{ .path = "test/system/services/input-source" },
+        .@"input-test" = .{ .path = "test/system/services/input-test" },
+        .@"args-echo" = .{ .path = "test/system/services/args-echo" },
+        .@"process-test" = .{ .path = "test/system/services/process-test" },
+        .@"thread-test" = .{ .path = "test/system/services/thread-test" },
         // See `zig fetch --save <url>` for a command-line interface for adding dependencies.
         //.example = .{
         //    // When updating this field to a new URL, be sure to delete the corresponding