You have to write the command like this I think:
/testforblock 30 56 -1124 minecraft:chest -1 {Items:[{Slot:0b,id:"minecraft:stone",Count:1b}]}
or
/testforblock 30 56 -1124 minecraft:chest 2 {Items:[{Slot:0b,id:"minecraft:stone",Count:1b}]}
Hope this helped!
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