Enforces static imports for JUnit 4 assertions and assumptions

This commit configures Checkstyle to enforces static imports for JUnit 4
assertions and assumptions.

See gh-22932

src/checkstyle/checkstyle.xml

@@ -97,7 +97,7 @@
 		<module name="com.puppycrawl.tools.checkstyle.checks.imports.IllegalImportCheck">
 			<property name="regexp" value="true" />
 			<property name="illegalClasses"
-				value="^reactor\.core\.support\.Assert,^junit\.framework\..+,^org\.junit\.Assert\.assertThat,^org\.junit\.Assume\.assumeThat,^org\.junit\.rules\.ExpectedException,^org\.slf4j\.LoggerFactory" />
+				value="^reactor\.core\.support\.Assert,^junit\.framework\..+,^org\.junit\.Assert$,^org\.junit\.Assume$,^org\.junit\.Assert\.assertThat,^org\.junit\.Assume\.assumeThat,^org\.junit\.rules\.ExpectedException,^org\.slf4j\.LoggerFactory" />
 		</module>
 
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