Add unit test to expect NoMethodError

Add a unit test that expects a `NoMethodError` exception to be
raised when trying to access an undefined test parameter on the
TestParameters class.

Also use `let` to dry up the existing tests.
Dynamically generate param attribute readers

Dynamically generate attribute reader methods for the test parameters
read in from the YAML-formatted configuration file.

Implementing these methods dynamically means that we don't need to
manually add a new method and bump the version of this gem each time we
change test parameters in the integration tests.

Note that we're using `send` because `define_method` is a private method
and we're calling it from inside the class initalizer.
Throw friendly error message if param undefined

Throw a friendly error message using `method_missing` if an undefined
method is called on the TestParameters class, which usually indicates
that an integration test is trying to use a test parameter that has not
been defined or added to the YAML-formatted configuration file consumed
by vCloud Tools Tester.

Add new instance variable, `@config_file`, so that we can access the
name of the configuration file and use it in the error message in
`method_missing`.

* * *

Also amends the test to check for the new error message. Because we need
to match the error string, we use `let` to define `config_file` in the
tests so that it can be accessed both from inside the test and by the
other `let` declaration.