Fix InferredStatementContext.isSupportedByFeatures()
StmtContextUtils.checkFeatureSupport() does not work for
InferredStatementContext, as it does not have any declared
substatements -- that only works for OriginalStmtCtx and its subclasses.
Split out computation of this statement's contribution to
isSupportedByFeatures() into a separate method -- allowing
{Inferred,Replica}StatementContext to forward to their prototype or
source.
This also flushes out a bad assertion in YT1393Test, as the second
augment now get correctly disabled.
JIRA: YANGTOOLS-1514
Change-Id: Icf9740cd828bd337b76f38cf33ee29d607f680aa
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>