Allow to run regression test against existing installation without recompiling.

Allow to run regression test against existing installation without recompiling.

It is possible to run regression test using existing installation.

regression.sh -m noinstall

However some of tests fail in this case because those tests require
pgpool to be compiled with variable HEALTHCHECK_DEBUG is set. This is
only possible in following procedure.

make clean
cd src/test/regression
./regress.sh

To run the regression test against existing installation new config
variable "health_check_test" is added. The source code is always
compiled as if HEALTHCHECK_DEBUG is set. The test facility is not
activated unless health_check_test is set to on.

For now I push to only master branch. After some tests, I am going to
push to all supported branches. I know adding new parameter to stable
branches is unusual but the feature is for enhancing test and it is
not visible to ordinal users. So I think my plan is justified by them.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2021-December/004078.html