[JENKINS-22214] Fix findings from FindBugs static analysis

- CppcheckMetricUtil: A boxed primitive is created from a String, just to extract the unboxed primitive value. It is more efficient to just call the static parseXXX method.
- CppcheckResult: Private method com.thalesgroup.hudson.plugins.cppcheck.CppcheckResult.getPreviousReport() is never called.
- CppcheckResult: Class com.thalesgroup.hudson.plugins.cppcheck.CppcheckResult defines non-transient non-serializable instance field cppcheckSourceContainer.
- CppcheckSource: Class com.thalesgroup.hudson.plugins.cppcheck.CppcheckSource defines non-transient non-serializable instance field cppcheckWorkspaceFile.
- AbstractWorkspaceTest: AbstractWorkspaceTest.workspace not initialized in constructor and dereferenced in com.thalesgroup.hudson.plugins.cppcheck.AbstractWorkspaceTest.deleteWorkspace().
- CppcheckParserResultTestCppcheckParserResultTest: AbstractWorkspaceTest.workspace not initialized in constructor and dereferenced in com.thalesgroup.hudson.plugins.cppcheck.CppcheckParserResultTest.testNoMatch().
- CppcheckParserTest: Usage of GetResource in com.thalesgroup.hudson.plugins.cppcheck.CppcheckParserTest.processCheckstyle(String, int, int, int, int, int, int) may be unsafe if class is extended.
- serialVersionUID member variables defined in serializable classes.
- Unneccessary @SuppressWarnings("unused") removed to solve compiler warnings.