Hide R namespaces for testing purposes

I'm developing an R package which can optionally use third-party packages, as specified in the "suggests" field of the DESCRIPTION file. Functions which may optionally use the suggested third-party packages check for their availability via the requireNamespace function. These functions behave differently depending on the presence or not of the suggested packages.

While implementing tests for theses functions, I would like to test for the case where the optional packages are not present. My question is: is it possible to temporarily hide a namespace for testing purposes, such that the requireNamespace function returns FALSE when searching for it?