Add test for Overlay Method Tables

[vchuravy]

Adds the test for #646 (still broken).

Of note is that in the REPL this works fine:

julia> Base.Experimental.@MethodTable(method_table)
# 0 methods for callable object

julia> foo() = 1
foo (generic function with 1 method)

julia> Base.Experimental.@overlay Main.method_table foo() = 2

julia> methods = Base._methods_by_ftype(Tuple{typeof(foo)}, method_table, 1, Base.get_world_counter())
1-element Vector{Any}:
 Core.MethodMatch(Tuple{typeof(foo)}, svec(), foo() @ Main REPL[3]:1, true)

julia> display(Base.uncompressed_ir(methods[1].method))
CodeInfo(
1 ─     return 2
)

julia> Base.Experimental.@overlay Main.method_table foo() = 3

julia> methods = Base._methods_by_ftype(Tuple{typeof(foo)}, method_table, 1, Base.get_world_counter())
1-element Vector{Any}:
 Core.MethodMatch(Tuple{typeof(foo)}, svec(), foo() @ Main REPL[6]:1, true)

julia> display(Base.uncompressed_ir(methods[1].method))
CodeInfo(
1 ─     return 3
)