Migrate Junit 4 to Junit 5: java-core

[blakeli0]

See this doc for important differences and benefits of Junit 5.
See the official doc from Junit5 for migration tips.

In general, for each and every unit test

• If the only change is the package name, change the import statement. For example, replace org.junit.Assert.assertEquals with org.junit.jupiter.api.Assertions.assertEquals.
• If the feature is still supported, convert the Junit 4 syntax to Junit 5 syntax. For example, replace @Before with @BeforeEach
• If the feature is not supported anymore, re-write the unsupported part with the alternative. For example, replace @Test(expected = …​) with assertThrows.

See example PR that adds Junit 5 dependencies to gax-java and migrates one test to Junit5.

[JoeWang1127]

Some downstream libraries, e.g., java-logging, depend on a test class, BaseSerializationTest, in java-core.

We should keep these tests as-is.