It is expected that under enable-private-import internal/private symbols
become public. So that symbol-diffing would fail. Disable symbol diffing
under that test mode.
rdar://51304243
A resilient change should not *remove* previously-public symbols.
Note that currently, the test_superclass_properties test does not
meet this critierion, because we always emit keypath property
descriptors, even when the property is an override. Fixing this
is a larger change that I'll address in a follow-on PR, so for now
I'm just going to disable the symbol check for this one test only.
Part of <rdar://problem/40432647>.
