`compress` test seems broken

[t-bltg]

Reproducer:

julia> using CSV
julia> default_table = (col1=[1,2,3], col2=[4,5,6], col3=[7,8,9])

julia> io = IOBuffer()
julia> CSV.write(io, default_table; compress=true)  # compression

julia> take!(io) |> println
UInt8[]  # nothing written !

julia> io = IOBuffer()
julia> CSV.write(io, default_table; compress=false)  # no compression

julia> take!(io) |> println
UInt8[0x63, 0x6f, 0x6c, 0x31, 0x2c, 0x63, 0x6f, 0x6c, 0x32, 0x2c, 0x63, 0x6f, 0x6c, 0x33, 0x0a, 0x31, 0x2c, 0x34, 0x2c, 0x37, 0x0a, 0x32, 0x2c, 0x35, 0x2c, 0x38, 0x0a, 0x33, 0x2c, 0x36, 0x2c, 0x39, 0x0a]

Encountered in #1176.
The test also seem to fail a pending previous PR: #1173, with the same error.

[t-bltg]

It seems the tests are no longer valid, this works (fix added to #1176):

using CSV, Test, Tables
const default_table = (col1=[1,2,3], col2=[4,5,6], col3=[7,8,9])

# uncompressed writing
io = IOBuffer()
CSV.write(io, default_table)
seekstart(io)  # <-- needed
ct = CSV.read(io, Tables.columntable)
@test ct == default_table

# compressed writing
mktemp() do path, io
    CSV.write(io, default_table; compress=true)
    # io is now closed
    ct = CSV.read(path, Tables.columntable)
    @test ct == default_table
end