Update to use the Test2 framework

[stevenl]

We need this because called_ok.t prints stuff out about "planned 0 tests" in the subtest even though the test passes. This is probably because "Test::Builder::Tester does not handle plans well" and there are no plans to fix it (see https://metacpan.org/pod/Test::Builder::Tester#BUGS). Its use is now discouraged in favour of Test2::API::intercept()

t/called_ok.t ............. 1/64     # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.
    # Looks like you planned 0 tests but ran 1.