qa: add numerous subtree test

qa: add numerous subtree test

When the ESubtreeMap is very large (~5k+ subtrees), the MDS will
end up logging only a few events (as bad as 1) per segment as the
subtree map dominates the segment size.

This test simply creates an artificially large subtree and confirms that
other file system activity completes in a timely manner. This is now
taking advantage of the minor segments which allows for a normal set of
events per log segment (and fewer subtree maps). The test fails on the
current main HEAD.

Historical note: when I first observed this abberant behavior, the
vstart cluster was actually using mds_debug_subtrees = True (the default
for every vstart cluster). This caused the MDS to write out the subtree
map (for debugging reasons) with every event. When testing the MDS with
large subtrees (distributed ephemeral pinning), this caused the MDS to
slow to a trickle of operations per second. Despite this unintentional
misconfiguration, the problem still exists but the number of auth
subtrees must be large for a particlar rank to replicate the behavior.

On main HEAD, the creation of 10k files (workload stage) takes ~110
seconds. On this branch, it takes ~30 seconds.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>