dtc: Don't use env(1) in testsuite

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> The run_tests.sh script currently invokes the testcase binaries via
> env(1).  This behaviour is inherited from the libhugetlbfs testsuite
> which uses this approach to easily set various configuration
> environment variables in testcases.
> 
> We don't use that for dtc, and are unlikely to ever want to.
> Therefore this patch removes that technique, which substantially
> speeds up the testsuite.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl