custom postgres launcher for tests

This patch allow to use custom postgres launcher for tests (tap&regress)
by setting environment variable PGLAUNCHER.

Other known methods (like: https://wiki.postgresql.org/wiki/Valgrind)
requires
to perform installation, build system modifications, executable replacement
etc...

And proposed way is simpler and more flexible.

** Use-case: run checks under Valgrind

- prepare launcher

echo 'exec valgrind postgres "$@"' > /tmp/pgvalgrind
chmod +x /tmp/pgvalgrind

- execute regress tests under Valgrind

PGLAUNCHER=/tmp/pgvalgrind TESTS=gin make check-tests

- execute concrete tap-test under Valgrind

PGLAUNCHER=/tmp/pgvalgrind PROVE_TESTS=t/001_stream_rep.pl make \
check -C src/test/recovery

** Use-case: execute tests with different postgres versions

- prepare multi-launcher

cat <<EOF > /tmp/launcher
cp -f `pwd`/src/backend/postgres.v* \`pg_config --bindir\`
exec postgres.v\$V "\$@"
EOF
chmod +x /tmp/launcher

- make some versions of postgres binary

./configure "CFLAGS=..." && make
mv src/backend/postgres src/backend/postgres.v1

./configure "CFLAGS=..." && make
mv src/backend/postgres src/backend/postgres.v2

- run checks with different postgres binaries

PGLAUNCHER=/tmp/launcher V=1 make check -C contrib/bloom
PGLAUNCHER=/tmp/launcher V=2 make check -C contrib/bloom