Technical debt has many different meaning. It depends on the context that it is used in. Wikipedia has a nice statement that I quite like.
“The debt can be thought of as work that needs to be done before a
particular job can be considered complete or proper. If the debt is not
repaid, then it will keep on accumulating interest, making it hard to
implement changes later on.” – 
Unit tests is one area where technical debt can be easily obtained,
and the amount of debt can grow exceptionally quickly. It definitely
needs to be forethought and well planned before it is used in full
force. The following questions must be answered to fully.
What language will the test code be implemented in? –
This is critical contributor to technical debt. If you use a language
which the majority of your development team is not familiar with, the
chances of increasing technical debt is quite high. As the language is
not well known by your team, training will need to be provided to it and
the future team that will be maintaining the code. For example,
implementing the test code in Java while the code under test is in C
means that your development team must know Java and C to deliver good
Is the infrastructure of your unit test environment well supported?
– Using a unit test framework that does not have a well supported
community (either through the online community or supported
commercially) means that the support of it must be carried out by your
development team. When the code enters maintenance phase, the unit test
framework still needs to be maintained, as specially as the computers
and operations system it is using starts to revised to newer versions.
This particularly true if the code has a long life time, as it is true
with most embedded software. As more test cases are being implemented,
the amount of technical debt gets larger and it makes supporting the
unit test framework quite difficult.
Is the unit test framework integrated well with your tool chain?
– Using a unit test framework that is not well integrated in your tool
chain means that the result from you test runs must be analysed
manually. For a small project, manual analysis of the the test result is
OK and relatively easy. However as the project gets bigger with a large
code set with high level complexity, good integration into your tool
chain is a must. This will ensure that the results are easily analysed,
and the correct notifications are generated. It will ensure continuous
integration can be achieved easily and automatically. It will also means
that removing manual work will also reduce the risk of performing the
manual process incorrectly.
By answering these questions, there is a chance that your technical debt could become technical credit.
I am writing a Guide to Successful Unit Tests, you can get it here at Leanpub.
 – https://en.wikipedia.org/wiki/Technical_debt