Allow tests to pass in OpenSSL FIPS mode

While working on the column encryption patch, I wanted to check that
what is implemented also works in OpenSSL FIPS mode. I tried running
the normal test suites after switching the OpenSSL installation to FIPS
mode, but that failed all over the place. So I embarked on fixing that.
Attached is a first iteration of a patch.

The main issue is liberal use of the md5() function in tests to generate
random strings. For example, this is a common pattern:

SELECT x, md5(x::text) FROM generate_series(-10,10) x;

This can be replaced by

SELECT x, encode(sha256(x::text::bytea), 'hex')
FROM generate_series(-10,10) x;

In most cases, this could be further simplified by not using text but
bytea for the column types, thus skipping the encode step.

Some tests are carefully calibrated to achieve a certain column size or
something like that. These will need to be checked in more detail.

Another set of issues is in the SSL tests, where apparently some
certificates are generated with obsolete hash methods, probably SHA1
(and possibly MD5 again). Some of this can be addressed by just
regenerating everything with a newer OpenSSL installation, in some other
cases it appears to need additional command-line options or a local
configuration file change. This needs more research. I think we should
augment the setup used to generate these test files in a way that they
don't depend on the local configuration of whoever runs it.

Of course, there are some some tests where we do want to test MD5
functionality, such as in the authentication tests or in the tests of
the md5() function itself. I think we can conditionalize these somehow.
That looks like a smaller issue compared to the issues above.