Migrate away from ExpectedException (#22922)

* Add limited checkstyles to test code

Add a limited set of checkstyle rules to the test codebase to improve
code consistency.

* Fix checksyle violations in test code

* Organize imports to fix checkstyle for test code

* Migrate to assertThatExceptionOfType

Migrate aware from ExpectedException rules to AssertJ exception
assertions. Also include a checkstyle rules to ensure that the
the ExpectedException is not accidentally used in the future.

See gh-22894

src/checkstyle/checkstyle.xml

@@ -94,6 +94,11 @@
 			<property name="option" value="bottom" />
 			<property name="sortStaticImportsAlphabetically" value="true" />
 		</module>
+		<module name="com.puppycrawl.tools.checkstyle.checks.imports.IllegalImportCheck">
+			<property name="regexp" value="true" />
+			<property name="illegalClasses"
+				value="^reactor\.core\.support\.Assert,^org\.junit\.rules\.ExpectedException,^org\.slf4j\.LoggerFactory" />
+		</module>
 
 		<!-- Javadoc Comments -->
 		<module name="com.puppycrawl.tools.checkstyle.checks.javadoc.JavadocTypeCheck">
@@ -145,6 +150,15 @@
 			<property name="illegalPattern" value="true" />
 			<property name="message" value="Trailing whitespace" />
 		</module>
+		<module
+			name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="maximum" value="0" />
+			<property name="format"
+				value="assertThatExceptionOfType\((NullPointerException|IllegalArgumentException|IOException|IllegalStateException)\.class\)" />
+			<property name="message"
+				value="Please use specialized AssertJ assertThat*Exception method." />
+			<property name="ignoreComments" value="true" />
+		</module>
 
 		<!-- Spring Conventions -->
 		<module name="io.spring.javaformat.checkstyle.check.SpringLambdaCheck">