Another round of portability hacking on ECPG regression tests.

Another round of portability hacking on ECPG regression tests.

Removing the separate Windows expected-files in commit f1885386f
turns out to have been too optimistic: on most (but not all!) of our
Windows buildfarm members, the tests still print floats with three
exponent digits, because they're invoking the native printf()
not snprintf.c.

But rather than put back the extra expected-files, let's hack
the three tests in question so that they adjust float formatting
the same way snprintf.c does.

Discussion: https://postgr.es/m/18890.1539374107@sss.pgh.pa.us