The -showGroups option relativizes group paths against the test suite, which makes groups like
  all = /
show up as:
  all:
I.e. look undistinguishable from empty groups. Indeed as space character is printed but to human eyes it's misleading.

I propose adding a special case in
https://github.com/openjdk/jtreg/blob/1a50034da41e1c10bffd69914c8970a5fd75aabb/src/share/classes/com/sun/javatest/regtest/tool/Tool.java#L1527
to print "/" groups as the character "/".