scripts: use CT_CONFIG_SHELL instead of CT_SHELL

CT_SHELL is undefined.
Thus, the generated wrapper scripts are not executable by the kernel
because they do not contain a valid interpreter.
Use CT_CONFIG_SHELL instead.

Signed-off-by: "Titus von Boxberg" <titus@v9g.de>
(transplanted from cf509170838f4e289fe25617e3f3db1d6d6ffc41)

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =