tests: bsim: Fix command execution with quoted string

When running a bsim test where an option's argument is a quoted string,
using $@ to execute the arguments of the Execute function causes
incorrect quote handling.

To correctly pass all arguments, add extra quotes around $@. This
ensures that quoted strings remain intact when passed to the executable.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>