Stop imports from one AST polluting checking of another AST

Fixes gh-110

Author: wilkinsona

spring-javaformat/spring-javaformat-checkstyle/src/main/java/io/spring/javaformat/checkstyle/check/SpringJUnit5Check.java

@@ -69,6 +69,7 @@ public int[] getAcceptableTokens() {
 
 	@Override
 	public void beginTree(DetailAST rootAST) {
+		this.imports.clear();
 		this.testMethods.clear();
 	}