Make echidna exercise-7 solution self contained

[glarregay-tob]

Removes the need for Etheno in Exercise 7

[CLAassistant]

All committers have signed the CLA.

[glarregay-tob]

Output:

Analyzing contract: solution.sol:E2E
testPoolBalance(): failed!💥  
  Call sequence:
    execute() Value: 0x1f7
    setEnableDeposit(true,500)
    flashLoan(1)
    setEnableDeposit(false,0)
    setEnableWithdraw(true)
    execute()
    testPoolBalance()

Event sequence: Panic(1)
setEnableWithdraw(bool):  fuzzing (17910/100000000000)
setEnableDeposit(bool,uint256):  fuzzing (17910/100000000000)
execute():  fuzzing (17910/100000000000)
flashLoan(uint256):  fuzzing (17910/100000000000)
AssertionFailed(..):  fuzzing (17910/100000000000)

[gustavo-grieco]

CI test for exercise 7 is failing with echidna-test: sideentrance.yaml: openBinaryFile: does not exist (No such file or directory). Let's fix it and merge on green.