scripts: twisterlib: display testplan build duration

Display how long it takes to build the testplan in the logs. This can
be useful since the duration to bulid the plan can vary wildly depending
on the arguments given. Without this information, it is difficult to
tell (from CI logs) whether a long execution time is from actually
building and running the tests, or just generating the testplan.

For example, in a downstream repo:
```
./zephyr/scripts/twister --integration -iv -T . --dry-run

INFO    - Building initial testsuite list...
INFO    - Built testsuite list in 10.58 seconds
```
vs
```
./zephyr/scripts/twister --integration -iv -T . --dry-run \
--vendor nordic --vendor zephyr

INFO    - Building initial testsuite list...
INFO    - Built testsuite list in 201.01 seconds
```

Signed-off-by: Jordan Yates <jordan@embeint.com>

Author: JordanYates

scripts/pylib/twister/twisterlib/testplan.py

@@ -16,6 +16,7 @@
 import re
 import subprocess
 import sys
+import time
 from argparse import Namespace
 from collections import OrderedDict
 from itertools import islice
@@ -830,6 +831,7 @@ def apply_filters(self, **kwargs):
         integration_mode_list = test_config_options.get('integration_mode', [])
 
         logger.info("Building initial testsuite list...")
+        build_list_start = time.time()
 
         keyed_tests = {}
         for _, ts in self.testsuites.items():
@@ -1220,6 +1222,9 @@ def apply_filters(self, **kwargs):
             change_skip_to_error_if_integration(self.options, inst)
             inst.add_missing_case_status(inst.status)
 
+        build_list_duration = time.time() - build_list_start
+        logger.info(f"Built testsuite list in {build_list_duration:.2f} seconds")
+
     def _find_required_instance(self, required_app, instance: TestInstance) -> TestInstance | None:
         if req_platform := required_app.get("platform", None):
             platform = self.get_platform(req_platform)