tests: remove extra indentation in exec block

tests: remove extra indentation in exec block

The exec block was indented by an extra space, causing

line 439, in parse_block_mapping_key
    "expected <block end>, but found %r" % token.id, token.start_mark)
yaml.parser.ParserError: while parsing a block mapping
  in "<string>", line 111, column 3:
      sequential:
      ^
expected <block end>, but found '<block sequence start>'
  in "<string>", line 126, column 4:
       - exec:
       ^

when trying to run upgrade/hammer-x

Reported-by: Yuri Weinstein <yweins@redhat.com>
Signed-off-by: Nathan Cutler <ncutler@suse.com>