[IR Framework] Allow IR tests to build and use Whitebox without -DSkipWhiteBoxInstall=true

Currently, the IR framework always tries to install the Whitebox by moving the Whitebox class file to the JTreg class path. However, when a test already builds the Whitebox and uses it as part of the test, we cannot access it on certain platforms. On Windows, for example, we'll get the following exception:

Caused by: java.nio.file.FileSystemException: sun\hotspot\WhiteBox.class: The process cannot access the file because it is being used by another process

To mitigate this problem, one can specify -DSkipWhiteBoxInstall=true which was already done in JDK-8283187. But this is not a good solution as the user should not be bothered with the inner workings of the IR framework.

I propose to get rid of this flag by reworking the Whitebox installation process.