Most of the research into hydration and testosterone comes from studies into post-exercise testosterone levels. Unfortunately, the relevance of these studies to testosterone levels in general is not really clear.
This study http://jap.physiology.org/content/105/3/816.long looked at men undergoing resistance training at different hydration levels - either well hydrated (euhydrated), or in two different levels of dehydration (or hypohydration as the study refers to it), 2.5% or 5%. Dehydration reduced the normal elevation of testosterone that occurs in response to exercise, but increased cortisol by a fair bit.
In a similar study on runners, https://www.ncbi.nlm.nih.gov/pubmed/17006802?access_num=17006802&link_type=MED&dopt=Abstract hydration didn't really have an effect on testosterone levels, but again increased cortisol.
In this study cortisol also rose here https://www.ncbi.nlm.nih.gov/pubmed/12471300?access_num=12471300&link_type=MED&dopt=Abstract
This study looking at low intensity exercise found that hydration had no effect on testosterone or cortisol. https://www.ncbi.nlm.nih.gov/pubmed/7851363
I'll need to go into this in a lot more depth to get some meaningful information!