pg_dump test: Make concatenated create_sql commands more readable

When the pg_dump 002_pg_dump.pl test generates the command to load the
schema, it does

# Add terminating semicolon
$create_sql{$test_db} .= $tests{$test}->{create_sql} . ";";

In some cases, this creates a duplicate semicolon, but more importantly,
this doesn't add any newline. So if you look at the result in either
the server log or in tmp_check/log/regress_log_002_pg_dump, it looks
like a complete mess. The attached patch makes the output look cleaner
for manual inspection: add semicolon only if necessary, and add two
newlines.