framework: fix bug in Test class

This adds a test and a fix for a bug in the way the Test class is
implemented. Currently Test stores the value passed to it for test
arguments, and mutates these. Now, since most tests currently use a
string which is an immutable object (changes are actually
re-assignment), this hasn't been a problem, but later in this series
this will cause subtle problems.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>