You dont really have enough info about your pom but this looks likely:
Javassist is a transitive dependency of Hibernate (see here), before you added powermock it was being included in your WAR file as a result of this dependency.
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>XXXXX</version>
</dependency>
With your powermock trickery you have told Maven that javassist should not be included in your WAR file by explicitly making it test scope with:
<dependency>
<groupId>org.javassist</groupId>
<artifactId>javassist</artifactId>
<version>3.18.1-GA</version>
<scope>test</scope>
</dependency>
So, change the javassist scope to runtime
and it will be included in your WAR file and your ClassNotFoundException
should go away.