pgsql: Build src/test/isolation during "make" and "make install".

Build src/test/isolation during "make" and "make install".

This hack closes a race condition in "make -j check-world" and "make -j
installcheck-world".  Back-patch to v10, before which these parallel
invocations had worse problems.

Discussion: https://postgr.es/m/20171106080752.GA1298146@rfd.leadboat.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/de0aca6a82af9c04cb4634d091ab065763fd4d5a

Modified Files
--------------
src/Makefile                | 1 +
src/test/isolation/Makefile | 7 +++++++
2 files changed, 8 insertions(+)