Initialize a test counter, a matrix used to tally tests, their stack depth
and the execution order as the trace associated with
key is hit. Each
test trace is an environment, which allows assignment into a pre-allocated
tests matrix with minimall reallocation.
tests matrix has columns
corresponding to the test index (the index of the associated test in
.counters$tests), the stack depth when the trace is evaluated and the
number of traces that have been hit so far during test evaluation.