Test that temp file has been deleted before triggering heap dump

The MBean requires that a file does not exist at the location to which
it’s dumping the heap. This commit updates the test to ensure that the
temporary file that’s created to get a location for the heap dump file
is deleted before asking the MBean to dump the heap to that location.

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/mvc/HeapdumpMvcEndpointTests.java

@@ -152,6 +152,9 @@ public class HeapdumpMvcEndpointTests {
 					if (TestHeapdumpMvcEndpoint.this.locked) {
 						throw new InterruptedException();
 					}
+					if (file.exists()) {
+						throw new IOException("File exists");
+					}
 					FileCopyUtils.copy(TestHeapdumpMvcEndpoint.this.heapDump.getBytes(),
 							file);
 				}