Changes to definitions of <testsuite> and <testcase> in JUnit reports

having the same problems as in:
https://github.com/beatfactor/nightwatch/issues/191

namely:
Treating each assertion as a testcase leads to 1) overflow of information in results, 2) CI systems not being able to track tests history properly

I suggest:
a single test file to be treated as a testsuite (java TestClass analogue), while testcase is a Test Method analogue. As each report contains only 1 testsuite, "testsuites" node can be omitted. 
<testcase> contains a number of performed assertions in the corresponding attribute.
In case of failure, a sequence of assertions up to the failed one is printed along with the stacktrace.