Don't try to test files named with a trailing dot on Windows

Don't try to test files named with a trailing dot on Windows

The pg_verify_checksums test tries to create files with corrupt data
named "123." and "123_." But on Windows a file name with a trailing dot
is the same as a file without the trailing dot. In the first case this
will create a file with a "valid" name, which causes the test to fail in
an unexpected way, and in the secongd case this will be redandant as the
test already creates a file named "123_".

Bug discovered by buildfarm animal bowerbird.