Replace the usage of PowerMock with Mockito in the Bookkeeper tests

[lhotari]

PowerMock is no longer maintained. The last release way in January 2020.
It should be replaced with Mockito. A similar replacement was done in Pulsar in these PRs:

• [test] Replace PowerMockito#mockStatic usages with mockito-inline pulsar#14098
• [test] remove PowerMockito from the classpath and replace the usages with Mockito pulsar#14250
• [improve][test] remove powermock-reflect dependency pulsar#17696

PowerMock / PowerMockito is holding back from upgrading to Mockito 4.11.0 . Mockito 4.11.x is the last version to support Java 8.
Getting rid of PowerMock will be needed for upgrading some other libraries for supporting newer JDK versions (Java 17 & Java 21).

[nodece]

Could you assign this issue to me?