Lakeshore340 - fix ioc tests

[FreddieAkeroyd]

The lakeshore 340 is regularly getting a test failure on debug builds.

  File "C:\Instrument\Apps\EPICS-System_Tests_debug\support\IocTestFramework\master\tests\lakeshore340.py", line 134, in test_WHEN_excitation_a_set_THEN_can_be_read_back
    self.ca.assert_setting_setpoint_sets_readback(excitation, "EXCITATIONA")
Expected PV, 'TE:SPARE70:LKSH340_01:EXCITATIONA' to have value 'Off' (type: 'str').
Final PV value was '30 nA' (type: 'str')

After investigation i believe the error is related to the use of dbpf in st.cmd to set THRESHOLDS:FILE - using dbpf processes the record and triggers the forward link, this makes the _CALC record send values using uninitialised data. What we really need to do is set a value in THRESHOLDS:FILE but not cause the chain to process as at that point we do not have e.g. a temperature setpoint.

The PRs below do this, they pass the macro directly into the db file so a default value is set. This would be easy to do if it was a stingin record, but i suspect dbpf was used as it is not possible to initialise a waveform like that. I have recoded to use a long string record that supports such initialisation.

To test

You may not get any failures on your computer, but confirm tests succeed. Run lakeshore system tests on iocTestFramework directory

[rerpha]

ran 10 iterations of the ioc tests and no failures so think this has fixed it.