chore: use assertEquals to improve test error message

[burkedavison]

Improve test error message to show expected and actual values on failure.

Example of unhelpful assertion message when using assertTrue: https://fusion2.corp.google.com/ci/kokoro/prod:cloud-devrel%2Fclient-libraries%2Fjava%2Fgoogle-auth-library-java%2Fnightly%2Fintegration/activity/210e3a70-92c9-4601-8bcb-49e2a9bf5753/log

[ERROR] com.google.auth.oauth2.PluggableAuthHandlerTest.getExecutableResponse_oidcResponse -- Time elapsed: 0.011 s <<< FAILURE!
java.lang.AssertionError
	at org.junit.Assert.fail(Assert.java:87)
	at org.junit.Assert.assertTrue(Assert.java:42)
	at org.junit.Assert.assertTrue(Assert.java:53)
	at com.google.auth.oauth2.PluggableAuthHandlerTest.getExecutableResponse_oidcResponse(PluggableAuthHandlerTest.java:744)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud