HADOOP-19257. S3A: ITestAssumeRole.testAssumeRoleBadInnerAuth failure (#7021)

Remove the error string matched on so that no future message change
from AWS will trigger a regression

Contributed by Steve Loughran

hadoop-tools/hadoop-aws/src/test/java/org/apache/hadoop/fs/s3a/auth/ITestAssumeRole.java

@@ -283,8 +283,7 @@ public void testAssumeRoleBadInnerAuth() throws Exception {
     conf.set(SECRET_KEY, "not secret");
     expectFileSystemCreateFailure(conf,
         AWSBadRequestException.class,
-        "not a valid " +
-        "key=value pair (missing equal-sign) in Authorization header");
+        "");
   }
 
   @Test