Fix some differnces in regression test.
1. result-convrsion-test.c
   Explicitly specify an appropriate time zone.
   isprint() doesn't work as expected in my environment. It's safe
   to verify that the parameter is a single byte before calling
   isprint().

2. prepare-test.c
   Explicitly specify "postgres_verbose" intervalstyle.

3. diagnostics.c
   pg_terminate_backend() sometimes succeeds with no error message.
Make it possible to build test suite on the folder other than the source one.