In tab-completion test, print out the value of TERM before changing it.

I'm curious to see what values are prevailing in the buildfarm.

Discussion: https://postgr.es/m/23181.1578167938@sss.pgh.pa.us

src/bin/psql/t/010_tab_completion.pl

@@ -38,6 +38,9 @@ $node->safe_psql('postgres',
 my $historyfile = "${TestLib::log_path}/010_psql_history.txt";
 $ENV{PSQL_HISTORY} = $historyfile;
 
+# Debug investigation
+note "TERM is set to '" . ($ENV{TERM} || "<undef>") . "'";
+
 # Ensure that readline/libedit puts out xterm escapes, not something else.
 $ENV{TERM} = 'xterm';