CI: split slow test mode into separate workflow #2

Job	Run time
CI slow full test mode / classicmode_1d_3dgrid testmode ON	15m 28s
CI slow full test mode / classicmode_3d testmode ON	23m 23s
CI slow full test mode / kilonova_1d_1dgrid testmode ON	10m 47s
CI slow full test mode / kilonova_1d_3dgrid testmode ON	11m 35s
CI slow full test mode / kilonova_2d_2dgrid testmode ON	19m 22s
CI slow full test mode / kilonova_2d_2dgrid_barnesthermalisation testmode ON	17m 24s
CI slow full test mode / kilonova_2d_2dgrid_expansionopac testmode ON	14m 48s
CI slow full test mode / kilonova_2d_2dgrid_xcomgammaphotoion testmode ON	16m 6s
CI slow full test mode / kilonova_2d_3dgrid testmode ON	19m 51s
CI slow full test mode / nebular_1d_3dgrid testmode ON	25m 52s
CI slow full test mode / nebular_1d_3dgrid_limitbfest testmode ON	27m 59s
CI slow full test mode / combine_checksums	0s
	3h 22m 35s

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