ArgumentHandler must use the same delimiters as in jvmti_tools.cpp

separating the commit from PR 370 / JDK-8252003: https://github.com/openjdk/jdk/pull/370/commits/5219b11650c00f9af5564758b3292458a48e7f21#r497658983

jvmti_tools.cpp supports ` `, `~` and/or `,` as delimiters[*] in -agentlib options, but nsk/share/jvmti/ArgumentHandler takes these options (via getAgentOptionsString) and then parse in parseOptionString, where StringTokenizer is used w/ default delimiters, ie \t\n\r\f, and as a result, it doesn't split options correctly in tests which use delimiters other than ' '. and in fact, nsk/jvmti/scenarios/contention/TC05/tc05t001 is using ',' and gets wrong options/values.

[*] https://github.com/openjdk/jdk/blob/master/test/hotspot/jtreg/vmTestbase/nsk/share/jvmti/jvmti_tools.cpp#L224