Remove --verbose from PROVE_FLAGS

Per discussion, the TAP tests are really more verbose than necessary, so
remove the --verbose flag from PROVE_FLAGS.  Also add comments to let
folks know how they can enable it if they really wish to, as suggested
by Craig Ringer.

Author: Michael Paquier, additional comments by me.
Discussion: https://postgr.es/m/CAMsr%2BYGAzcMDOZ_BirnMCL6Sb%3DMUjP0FRE82YBDSbXcf6pm9Yg%40mail.gmail.com

src/Makefile.global.in

@@ -339,7 +339,8 @@ PROVE = @PROVE@
 # There are common routines in src/test/perl, and some test suites have
 # extra perl modules in their own directory.
 PG_PROVE_FLAGS = -I $(top_srcdir)/src/test/perl/ -I $(srcdir)
-PROVE_FLAGS = --verbose
+# For more info, add to your make line: PROVE_FLAGS='--verbose'
+PROVE_FLAGS =
 
 # prepend to path if already set, else just set it
 define add_to_path

src/test/perl/README

@@ -20,6 +20,11 @@ src/test/ssl, or should be added to one of the suites for an existing utility.
 Note that all tests and test tools should have perltidy run on them before
 patches are submitted, using perltidy --profile=src/tools/pgindent/perltidyrc
 
+By default, to keep the noise low during runs, we do not set any flags via
+PROVE_FLAGS, but this can be done on the 'make' command line if desired, eg:
+
+make check-world PROVE_FLAGS='--verbose'
+
 Writing tests
 -------------