Fix "make check" to test headers (#1463)

Fix "make check" to test headers (#1463)

A "make check" testHeaders target is supposed to check that C/C++ header
files can be compiled "autonomously" (i.e. without any other code except
the required squid.h). Since 2010 commit a0fdc9b, existing testHeaders
were not detecting problems in any headers due to a use of a shell
heredoc with echo, a command that does not read from standard input.

This fix ensures that all C/C++ header files used by "make" are tested
in the corresponding "make check".

This change also improves parallel testing of individual header files.