Backport of #48452.
The SAML tests have large XML documents within which various parameters
are replaced. At present, if these test are auto-formatted, the XML
documents get strung out over many, many lines, and are basically
illegible.
Fix this by using named placeholders for variables, and indent the
multiline XML documents.
The tests in `SamlSpMetadataBuilderTests` deserve a special mention,
because they include a number of certificates in Base64. I extracted
these into variables, for additional legibility.
30389c6660