Update muon trigger ART test ro use stdcmath instead of imf

Disabling the --imf option in the muon trigger ART test to try to disentangle some irreproducible effects in the muon triggers (ATR-21879).

Trigger/TrigValidation/TriggerTest/test/test_trig_mc_v1Dev_slice_muon_grid.py

@@ -25,6 +25,7 @@ ex.type = 'athena'
 ex.job_options = 'TriggerJobOpts/runHLT_standalone.py'
 ex.input = 'ttbar'
 ex.threads = 1
+ex.imf = False
 # LS2_v1 soon to be renamed to Dev_pp_run3_v1
 ex.args = '-c "setMenu=\'LS2_v1\';doEmptyMenu=True;doMuonSlice=True;doWriteBS=False;doWriteRDOTrigger=True;"'