Update README.tests

document make check, make valgrind and fix a typo pointed out by
Daniel Neel <dneelyep@gmail.com>
Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=617019

Daniel

README.tests

@@ -14,17 +14,26 @@ program has a different testing purpose:
 
 The command:
 
+  make check
+or
   make -f Makefile.tests check
 
 should be sufficient on an Unix system to build and exercise the tests
 for the version of the library installed on the system. Note however
 that there isn't backward compatibility provided so if the installed
-version is older to the testsuite one, failing to compile or run the tests
+version is older than the testsuite one, failing to compile or run the tests
 is likely. In any event this won't work with an installed libxml2 older
 than 2.6.20.
+
 Building on other platforms should be a matter of compiling the C files
 like any other program using libxml2, running the test should be done
 simply by launching the resulting executables.
 
+Also note the availability of a "make valgrind" target which will run the
+above tests under valgrind to check for memory errors (but this relies
+on the availability of the valgrind command and take far more time to
+complete).
+
 Daniel Veillard
-Thu Jul 24 2008
+Mon May  7 2012
+