Allows passing regexps per subtest to --test.run and --test.bench
Note that the documentation explicitly states that the split regular
expressions match the correpsonding parts (path components) of
the bench/test identifier. This is intended and slightly different
from the i'th RE matching the subtest/subbench at the respective
level. Picking this semantics allows guaranteeing that a test or
benchmark identifier as printed by go test can be passed verbatim
(possibly quoted) to, respectively, -run or -bench: subtests and
subbenches might have a '/' in their name, causing a misaligment if
their ID is passed to -run or -bench as is.
This semantics has other benefits, but this is the main motivation.