corrected contract name for OracleLibTest

[ShayanShamsi]

Since here we are testing OracleLib.sol and we already have a contract named DSCEngineTest in DSCEngineTest.t.sol that is testing DSCEngine.sol, I thought it would be more appropriate if this test contract was named OracleLibTest instead of DSCEngineTest.

[PatrickAlphaC]

Ah great addition! Thank you!