Bug 5237: "make check" fails when ${TRUE} is a multicall binary (#2442)

Bug 5237: "make check" fails when ${TRUE} is a multicall binary (#2442)

testHeaders copies the 'true' binary to a file named testHeaders relying
on the fact that GNU's true always exits 0. When 'true' implementation
is a multicall binary (e.g., in rust-coreutils and on Alpine Linux), it
cannot be called by other name. Create an empty executable file instead.

squid-conf-tests also copied the 'true' binary, but that Makefile target
is never executed and, hence, should not create an executable file.