tests: counter: add mcux LPTMR alarm mode coverage

Extend the counter_basic_api tests with a dedicated configuration that
enables the MCUX LPTMR driver in alarm mode.

The new testcase:
- Selects CONFIG_COUNTER_MCUX_LPTMR_ALARM
- Runs the existing counter_basic_api suite against nxp_lptmr devices
  when built for a board that instantiates the LPTMR

This verifies that the alarm-only LPTMR configuration passes the common
counter API tests without relying on the TOP callback feature.

Signed-off-by: Holt Sun <holt.sun@nxp.com>

Author: Holt-Sun

tests/drivers/counter/counter_basic_api/testcase.yaml

@@ -51,3 +51,12 @@ tests:
       - CONFIG_TEST_DRIVER_COUNTER_MCUX_LPC_RTC_HIGHRES=n
     extra_args:
       DTC_OVERLAY_FILE="enable_standby.overlay"
+  drivers.counter.basic_api.mcux_lptmr_alarm:
+    tags:
+      - drivers
+      - counter
+    filter: dt_compat_enabled("nxp,lptmr")
+    depends_on: counter
+    timeout: 600
+    extra_configs:
+      - CONFIG_COUNTER_MCUX_LPTMR_ALARM=y