Unit tests provide examples for the semantics of an API, hence they can be seen as part of the API's documentation. So when a change in a unit test shows a change of the API's semantics, this can give the impression the changed unit test should trigger a bump of the version number.
However, that impression is wrong, since it mixes up cause and effect:
when the semantics of a component's API changes, then the component itself must have been changed, which causes the version number to be increased.
the necessary change of the unit test is merely a consequence of this.
when you add or change a unit test without changing anything else in the component's API, then you either fix a bug in the unit tests, or you document a certain behaviour which was already in the component.
Of course, when your distribution package includes all of the unit tests, and only the unit tests have changed from one release to the next, the distribution package should be given a new patch version number, as Thomas Owens wrote. This makes it clear that the distribution package contains changed content, howeverstill the API has not been changed.