doc: quote ** in test invocation example

[gthb]

The ** gets expanded in some shells (zsh), so protect it from that, else the invocation may fail, e.g.:

$ mvn -Dtest=javascript.** test
zsh: no matches found: -Dtest=javascript.**

... and a hasty person such as myself might miss the zsh in that output line and think the “no matches” is from Maven meaning that there are no tests identified by javascript.**, but there are.