following commit e545a463.

This is not specific to Visual C++ under MS Windows, but due to a bug
in GMP's vsnprintf replacement (now fixed in its development branch).

* INSTALL: moved and updated text about failures in the testsuite.
  New section: "tfprintf and tsprintf failures in check_null".
* doc/README.dev: updated MPFR_TESTS_SKIP_CHECK_NULL description.

(cherry picked from commit e6635fb4)