BGSCR - improve tests

```
[2026-08-12T10:35:31.098Z] Testing module tests.bgrscrpt in Device sim mode.
[2026-08-12T10:35:31.098Z] St.cmd path not found: 'C:\Instrument\Apps\EPICS\ioc\master\BGRSCRPT\iocBoot\iocBGRSCRPT-IOC-01\st.cmd'
[2026-08-12T10:35:31.098Z] No existence PV specified.
[2026-08-12T10:35:31.098Z] Starting IOC (BGRSCRPT_01), IOC log file is C:\Instrument\Var\logs\IOCTestFramework\log_tests_bgrscrpt_devsim_BGRSCRPT_01_ioc.log
[2026-08-12T10:35:43.277Z] IOC started, connecting to procserv pids 16456 at telnet port 62299
[2026-08-12T10:35:49.834Z] Press Ctrl-C to terminate tests, but be patient for IOC cleanups to run
[2026-08-12T10:35:49.834Z]
[2026-08-12T10:35:49.834Z] Running tests...
[2026-08-12T10:35:49.834Z] ----------------------------------------------------------------------
[2026-08-12T10:35:49.835Z] test_WHEN_script_from_path_macro_called_THEN_temp_directory_created (tests.bgrscrpt.BgrscrptTests.test_WHEN_script_from_path_macro_called_THEN_temp_directory_created) ... ok (0.001s)
[2026-08-12T10:35:49.835Z]
[2026-08-12T10:35:49.835Z] ----------------------------------------------------------------------
[2026-08-12T10:35:49.835Z] Ran 1 test in 0.001s
[2026-08-12T10:35:49.835Z]
[2026-08-12T10:35:49.835Z] OK
[2026-08-12T10:35:49.835Z]
[2026-08-12T10:35:49.835Z] Generating XML reports...
[2026-08-12T10:35:49.835Z] Generated XML report: test-reports\TEST-tests.bgrscrpt.BgrscrptTests-20260812113530.xml
[2026-08-12T10:35:49.835Z]
[2026-08-12T10:35:49.835Z] Terminating IOC (BGRSCRPT_01)
```
IOC log said `[2026-08-12 11:35:47] macLib: macro IOC is undefined (expanding string $(IOC))` test passed, but bgscript wasn't working properly on instruments. Need to catch this as it was an issue with loading macros via icpconfig.

Start with tests.bgrscrpt.BgrscrptTests and run the tests in Device sim mode, using the shown IOC log as the failure signal. Trace how macros are loaded via icpconfig and compare that with the existing script-from-path test. Done means the test suite detects the undefined IOC macro condition rather than passing while the bgscript is not working properly on instruments.

Written by the indexing model from the issue text.