SIGPIPE in TAP tests

Two buildfarm runs[1][2] from the last 90 days have failed in
src/test/authentication, like this:

  t/001_password.pl .. 
  Failed 3/8 subtests 
  t/002_saslprep.pl .. ok     1815 ms ( 0.00 usr  0.00 sys +  0.89 cusr  0.26 csys =  1.15 CPU)

  Test Summary Report
  -------------------
  t/001_password.pl (Wstat: 13 Tests: 5 Failed: 0)
    Non-zero wait status: 13
    Parse errors: Bad plan.  You planned 8 tests but ran 5.
  Files=2, Tests=17,  3 wallclock secs ( 0.04 usr  0.00 sys +  1.67 cusr  0.50 csys =  2.21 CPU)
  Result: FAIL

Compared to a good run, the other logs just end suddenly after the expected
"FATAL:  password authentication failed".  "Wstat: 13" means the Perl process
died to signal 13 (SIGPIPE).  This test invokes psql in a way that fails
authentication, and it writes "SELECT 1" to psql's stdin.  The SIGPIPE happens
if the psql process exits before that write.  I can reproduce this
consistently by patching a delay into IPC::Run:

--- IPC/Run.pm~    2017-05-12 06:46:43.000000000 -0700
+++ IPC/Run.pm    2017-12-09 00:24:06.876132628 -0800
@@ -1449,6 +1449,8 @@
 sub run {
     local $in_run = 1;     ## Allow run()-only optimizations.
     my IPC::Run $self = start(@_);
+    use Time::HiRes 'usleep';
+    usleep 100_000;

The two src/test/authentication tests then fail, but nothing else fails.
Let's ignore SIGPIPE in all TAP tests, which leaves some evidence in
regress_log_001_password:

  ack Broken pipe: write( 13, 'SELECT 1' ) at /home/nm/sw/cpan/lib/perl5/IPC/Run/IO.pm line 549.

To fix the actual failures, we can cease sending "SELECT 1"; it's enough to
disconnect immediately.  Patch attached.

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=crake&dt=2017-11-23%2008%3A37%3A17
[2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mandrill&dt=2017-10-30%2000%3A14%3A09