tests: build_all: Add net tag to the modem driver test

The modem driver test enables the networking stack, and some boards like
intel_adsp.* use this tag exclude networking tests.

Signed-off-by: Maureen Helm <[email protected]>

Author: MaureenHelm

tests/drivers/build_all/modem/testcase.yaml

@@ -1,6 +1,6 @@
 common:
   build_only: true
-  tags: drivers
+  tags: drivers net
 tests:
   drivers.modem.build:
     extra_args: CONF_FILE=modem.conf