Prior to this commit, @SpringJUnitConfig and @SpringJUnitWebConfig
did not declare `loader` attributes that alias @ContextConfiguration's
`loader` attribute. Consequently, it was not possible to configure a
custom ContextLoader via those annotations.
The lack of those `loader` attributes was an oversight, and this commit
introduces them to support custom ContextLoader configuration directly
via the @SpringJUnitConfig and @SpringJUnitWebConfig annotations.
Closes gh-31498