[SPARK-23892][Test] Improve converge and fix lint error in UTF8String-related tests

[kiszk]

What changes were proposed in this pull request?

This PR improves test coverage in UTF8StringSuite and code efficiency in UTF8StringPropertyCheckSuite.

This PR also fixes lint-java issue in UTF8StringSuite reported at here

[ERROR] src/test/java/org/apache/spark/unsafe/types/UTF8StringSuite.java:[28,8] (imports) UnusedImports: Unused import - org.apache.spark.unsafe.Platform.

How was this patch tested?

Existing UT

[kiszk]

ping @ueshin@hvanhovell@cloud-fan

[SparkQA]

Test build #89006 has finished for PR 21000 at commit 367dd4f.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[kiszk]

retest this please

[SparkQA]

Test build #89008 has finished for PR 21000 at commit 367dd4f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[kiszk]

Or, is it better to change from

assertTrue(s1.startsWith(s1));
assertTrue(s1.endsWith(s1));

to

assertTrue(s1.startsWith(s1));
assertTrue(s1.endsWith(s1));
assertTrue(s2.startsWith(s2));
assertTrue(s2.endsWith(s2));
assertTrue(s1.startsWith(s2));
assertTrue(s1.endsWith(s2));
assertTrue(s2.startsWith(s1));
assertTrue(s2.endsWith(s1));

?

[HyukjinKwon]

I think it's fine. LGTM

[hvanhovell]

LGTM - merging to master. Thanks!