Test identifiers conformity

Make sure the basic identifiers (that is the table name and the equality field name) are valid identifiers in terms of string length and character set.
Using "Database" in the assertion name is misleading. Nothing databases specific is tested. It's a very, very basic check for all kinds of identifiers, may it be a variable name in any programming language you can think of, index names, error codes and so on.

Also using "Is" ans "Valid" is completely unusual for PHPUnit assertions. There is not a single assertion that uses this naming scheme.

I just followed the given naming scheme (`assertEmpty`, `assertCount`, `assertJson`).