test(pdo_pgsql): Exclude pdo implicitly required by pdo_pgsql from EXTENSIONS

[KentarouTakeda]

The EXTENSIONS written in pdo_pgsql tests are:

• Both pdo and pdo_pgsql are declared
• Only pdo_pgsql is declared

Both were mixed. However, if pdo_pgsql is installed, pdo will be present, so there is no need to specify both.

Therefore, I unified all tests to declare only pdo_pgsql.

[devnexen]

makes sense to me

[cmb69]

Note that the --EXTENSION-- section is not only used to check whether extensions are loaded, but also to load them if available as shared module. So, if pdo can be built as shared module (I'm not sure if it can), both would need to be explicitly listed (probably in the order pdo, pdo_pgsql).