[SMALLFIX] Use static imports for standard test utilities in /alluxio/core/common/src/test/java/alluxio/util/ShellUtilsTest.java

[newnius]

Remove

import org.junit.Assert;

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.

[yangjf2019]

Hi, @newnius , Thank you very much . /assign @yangjf2019

[yangjf2019]

Here is PR.