scripts: allow logging of commands with variables

In a lot of places, we need to call some commands with specific
variable settings, a-la:
var1=val1 var2=val2 /foo/bar/buz opt1 opt2

Unfortunately, we currently can not log the variable settings.

Enhance CT_DoExecLog with a crude heuristic that works pretty well
and that can also log setting variables.

Reported-by: ANDY KENNEDY <ANDY.KENNEDY@adtran.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # 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 =