Test non-JSON phase encoding import/export

[Lestropie]

In addition to being embedded within a JSON, phase encoding information can additionally be imported or exported as:

• A single text file, containing an axis direction and total readout time per volume;
• A pair of files, the first containing one row per unique phase encoding direction & total readout time and the second indexing each volume into that file.
  While I'm more confident now that these should do as intended given the generality of the code in DWI metadata handling fixes MRtrix3/mrtrix3#2917, would nevertheless be preferable to add those imports / exports to the tests.