Ensure translatable strings, and string domains are not dynamic

@westonruter 
Couple more unit tests:
~~~php
__( 'string' . $var, 'domain' ); // Bad, should use variable for string

__( $var . 'string', 'domain' ); // Bad, should use variable for string

__( SOMETHING, 'domain' ); // Bad, should use constant for string

__( 'string' . SOMETHING, 'domain' ); // Bad, should use constant for string

__( SOMETHING . 'string', 'domain' ); // Bad, should use variable for string

__( 'string', $domain ); // Bad, should use variable for domain

__( 'string', 'my' . $domain ); // Bad, should use variable for domain

__( 'string', $domain . 'domain' ); // Bad, should use variable for domain
~~~