sample: uart passthrough: fix filtering and twister file

sample.yml is wrong and was not scanned. Also fix filters in the file so
it can build.

Signed-off-by: Anas Nashif <[email protected]>

Author: nashif

samples/drivers/uart/passthrough/sample.yaml

@@ -0,0 +1,10 @@
+sample:
+  name: UART Passthrough
+tests:
+  sample.drivers.uart.passthrough:
+    platform_allow:
+      - nucleo_l476rg
+    tags:
+      - serial
+      - uart
+    harness: keyboard