Changes

chore(pipelines/pingcap/tiflash): update context names for merged unit and build tests (#3551)

This commit modifies the context names for the merged unit test and
merged build jobs in the latest-postsubmits.yaml file, changing them
from 'wip/merged-unit-test' and 'wip/merged-build' to 'merged-unit-test'
and 'merged-build' respectively. This change is part of the ongoing
adjustments to streamline the CI process.
 (commit: 2a3fc42)