Add tests for protocols with multiple time units specifications

[MichaelClerx]

Add some tests for edge cases:

• Multiple independent var units declarations, with same units --> error?
• Multiple independent var units declarations, with different units --> error
• Time units set via independent var units and input/output, but OK --> not sure
• Time units set via independent var units and input/output, but conflicting --> error
• All of the above, but with part of the declarations in nested protocols

[jonc125]

• Multiple independent var units declarations, with same units --> error?

Probably not, because that would most commonly arise in nested protocols.

• Time units set via independent var units and input/output, but OK --> not sure

Think this is fine too. Redundant info, but that shouldn't prevent the protocol running.

• All of the above, but with part of the declarations in nested protocols

The outcome should be the same. So can check just after interfaces are merged.

[MichaelClerx]

See also #136

[MichaelClerx]

Covered in #158