[SMALLFIX] Use static imports for standard test utilities in alluxio/core/base/src/test/java/alluxio/collections/DirectedAcyclicGraphTest.java

[newnius]

Change

import org.junit.Assert;

to

import static org.junit.Assert.assertTrue;
import static org.junit.Assert.assertEquals;

AND

Update all

Assert.{{ method }}*

to

{{ method }}*

For example

update

    Assert.assertTrue(mGraph.getRoots().isEmpty());

to

    assertTrue(mGraph.getRoots().isEmpty());

[newnius]

Put the static imports before non-static imports.