The two shared DM/GM subtests in TestCreatePost rely on
FeatureFlags.EnableSharedChannelsDMs being false, but the config store
reapplies MM_FEATUREFLAGS_* env overrides on every Set, so an
UpdateConfig pin gets clobbered. Use t.Setenv to force the flag false
for the duration of each subtest, and drop the parent's
mainHelper.Parallel(t) so Go's "no Setenv under a parallel ancestor"
rule is satisfied. Sibling subtests still parallelize via their own
mainHelper.Parallel(t) calls.