Generate more customizable toml blocks for testing

[jamesray1]

For future extensibility with testing it seems like it would be helpful to generate more customizable tomls, by passing fields or using defaults.

In the case of #1775, the scenario here is that options include:

• create a pub fn instance1_with_file_storage(), which is like instance1, just with type = 'file' and path = 'TBD'. Not very elegant
• create a toml or toml block from scratch. Again, pretty verbose.
• create a instance_custom with optional arguments for each field and defaults.

By extension, this could be extended to other more complex toml block creators that have multiple fields in a block.

Originally posted by @jamesray1 in #1775 (comment)

[jamesray1]

OTOH, maybe this is overcomplicating things and it would be simpler for #1775 to just create a custom toml block.

[jamesray1]

From #1775 (comment):

More details:

create_test_conductor is used in test_remove_instance_clean_true. create_test_conductor uses test_toml, which in turn uses instance1(). And, as explained above, instance1() uses the memory variant of storage, without a persistent path. So in order to create a proper test_remove_instance_clean_true, that will actually be able to check whether the storage directory is removed, I need a different toml. To do that, it seems like it would be best to be able to pass a different instnace block to what are currently created with test_toml and instance1. For just this PR, the easiest way seems like to just add an option to select whether to use a memory type, with no path, or a file type, with some path relative to the config path, and then adjust wherever instance1() is called.

pub fn test_toml(test_name: &str, port: u32) -> String {
        let mut toml = header_block(test_name);

        toml = add_block(toml, agent1());
        toml = add_block(toml, agent2());
        toml = add_block(toml, dna());
        toml = add_block(toml, instance1());
        toml = add_block(toml, instance2());
        toml = add_block(toml, interface(port));
        toml = add_block(toml, logger());
        toml
    }

    pub fn instance1() -> String {
        r#"[[instances]]
agent = 'test-agent-1'
dna = 'test-dna'
id = 'test-instance-1'

[instances.storage]
type = 'memory'"#
            .to_string()
    }

[jamesray1]

If this issue reoccurs we can reopen it but for #1775 I've made a workaround.