Some tests were using functions curlx_tvnow and curlx_tvdiff which are not

Some tests were using functions curlx_tvnow and curlx_tvdiff which are not
part of the official libcurl API http://curl.haxx.se/lxr/source/lib/README.curlx
The documented way of using them would be to use timeval.c as a source code file.

The above described method works very well when statically linking libcurl and
apps, test programs, but has several drawbacks when you build a true shared
libcurl (i.e. Name space clash at linkage stage as functions are defined more
than once. Windows makefiles are not capable of handling this system of
source-level sharing)

So...

Now testutil.h and testutil.c define and implement tutil_tvnow and tutil_tvdiff
which replace curlx_tvnow and curlx_tvdiff for the libtest programs. Doing this
we avoid the above described problems, and the code in the testsuite does not
impose the need to keep those functions public in libcurl even when not part of
the API.