[FIXED JENKINS-20344] - SVN_REVISION is not exported if server url ends ...

...by / with further @HEAD suffix

SvnHelper function also truncates ending slashes => hash search works correctly. Also added some tests...

Resolves https://issues.jenkins-ci.org/browse/JENKINS-20344

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>