HBASE-29213 Fix timing in TestRegionNormalizerWorker.testRateLimit

[Sigma-Ma]

What changes were proposed in this pull request?

Start measuring elapsed time before constructing RegionNormalizerWorker.

Why are the changes needed?

The worker creates its RateLimiter during construction. Permits accumulated before the previous start timestamp could make the measured duration fall below five seconds and cause a false test failure.

How was this patch tested?

mvn -B -ntp -pl hbase-server -Dtest=TestRegionNormalizerWorker -Dsurefire.failIfNoSpecifiedTests=true -Dsurefire.rerunFailingTestsCount=0 -Dsurefire.skipSecondPart=true test

for run in {1..10}; do 
    mvn -B -ntp -pl hbase-server '-Dtest=TestRegionNormalizerWorker#testRateLimit' -Dsurefire.failIfNoSpecifiedTests=true -Dsurefire.rerunFailingTestsCount=0 surefire:test || break 
done