Include all exported functions in Test.jl documentation

[tkf]

While writing #37360, I noticed that not all exported names are included in Test documentation. This patch fixes it.

Exported names

julia> join(stdout, sort!(string.(names(Test))), "\n")
@inferred
@test
@test_broken
@test_deprecated
@test_logs
@test_nowarn
@test_skip
@test_throws
@test_warn
@testset
GenericArray
GenericDict
GenericOrder
GenericSet
GenericString
Test
TestSetException
detect_ambiguities
detect_unbound_args

With this PR:

$ cd stdlib/Test/docs/src

$ grep '^Test\.' index.md | sed 's/^Test\.//g' | LANG=C sort
@inferred
@test
@test_broken
@test_deprecated
@test_logs
@test_nowarn
@test_skip
@test_throws
@test_warn
@testset
GenericArray
GenericDict
GenericOrder
GenericSet
GenericString
TestSetException
detect_ambiguities
detect_unbound_args
finish
get_testset
get_testset_depth
record

The first list (except Test) is a subset of the second list.