ATest: Correct regex for finding aggregate methods

Original regex will get empty string in some cases, this CL make sure
that only non-empty string will be treat as test methods.

Bug: 211038991
Test: atest-src open-fixed-calculator --tf-template \
      metric_post_processor=google/template/postprocessors/metric-file-aggregate.xml \
      --aggregate-metric-filter cold_startup_com.google.android.calculator-mean \
      --aggregate-metric-filter cold_startup_com.google.android.calculator-max \
      -- --module-arg open-fixed-calculator:instrumentation-arg:iterations:=3
Change-Id: I898404cc69c7cd4bf1b5bc61a81f4d08f80231c4
1 file changed