Fix e2e issues

[fvictorio]

Our e2e tests have a several issues:

• The shebangs are wrong
• The temporary tests should be created outside the repository (for example, using mktemp -d, although I think /tmp is valid in the three runners used by the CI)
• npx is always used, but it should only be used for npm. yarn and pnpm should use their own versions (which is just the same command; that is: npx hardhat, yarn hardhat and pnpm hardhat)
• After fixing the shebangs, bash will be always used. This means that echo might not interpret \n (it doesn't do it in the CI nor on my machine), so we should use printf instead.
• We should add an assertion that no spurious, empty files are created in the root of the project. This function should work I think:

no_empty_files() {
  for file in $(find . -maxdepth 1 -type f); do
    if [ ! -s $file ]; then
      echo "File $file is empty"
      exit 1
    fi
  done
}

• Spend some time investigating why Windows e2e tests take way longer than Ubuntu e2e tests

When I fixed some of these things (not sure which one), the CI started failing for windows+pnpm, so we should look into that.