Add tests for miscellaneous functionality
Add tests for miscellaneous functionality

This includes:
- throwing errors for wong delimiters
- using the g:operator#substitute#*-variables
- changing the delimiter on the fly
- retrieving the last input-string using <Up> on input-dialog
- changes in /-register after substitution
- no change in cursor position after substitution
- no change in <,>-marks after substitution

Two tests are skipped because vspec freezes on them.
One test (<,>-marks not changing on ss) is skipped because
vim-operator-user always changes the >-mark-position on a linewise
operation.

milsen/vim-operator-substitute