CakePHP Fixure Does Not Generate Tables

UnitTest with CakePHPv2.5.5.

configuration

databases.php

public$xxxx=array(
    // ...
);
public$test_xxxx=array(
    'datasource' = > 'Database/MysqlExportDb',
    'persistent' = > false,
    'host' = > 'localhost',
    'login' = > 'root',
    'password' = > 'root',
    'database' = > 'test',
    'prefix' = >',
    'encoding' = > 'utf8',
);

Fixure Definition

class KeywordFixture extensions CakeTestFixture
{
    public$useDbConfig='test_xxxx';

    public$fields=array(...);

    public$records=array(...);
}

Test Code Side

class ShellTest extensions CakeTestCase
{
    public$fixes=array(
        'plugin.search_export.keyword',
    );
...

Symptom 1

If you run the test without the keywords table present, the following error occurs:

MissingTableException: Table keywords for model keyword was not found in datasource test_xxxx.

Symptom 2

If you manually create and run the keywords table, the keywords table disappears after the test runs correctly.

Question

As the table disappeared in phenomenon 2, we believe that Fixure is recognized in some way.
However, I do not know why the table is not generated.
What are the possible causes?

https://book.cakephp.org/2.0/ja/development/testing.html#id15