Bump openzeppelin-solidity from 2.5.0 to 3.2.0 in /testnet-contracts

Bumps [openzeppelin-solidity](https://github.com/OpenZeppelin/openzeppelin-contracts) from 2.5.0 to 3.2.0.
- [Release notes](https://github.com/OpenZeppelin/openzeppelin-contracts/releases)
- [Changelog](https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/CHANGELOG.md)
- [Commits](OpenZeppelin/openzeppelin-contracts@v2.5.0...v3.2.0)

Signed-off-by: dependabot[bot] <support@github.com>

testnet-contracts/package.json

@@ -27,7 +27,7 @@
     "chai-as-promised": "^7.1.1",
     "chai-bignumber": "^3.0.0",
     "ganache-cli": "^6.10.2",
-    "openzeppelin-solidity": "^2.1.3",
+    "openzeppelin-solidity": "^3.2.0",
     "truffle": "^5.1.46",
     "web3": "^1.2.6",
     "web3-utils": "^1.3.0"