Test the copy constructor's iterative path in CI

The copy constructor descends into 128 levels before it finishes a value
without the call stack, so the iterative path is otherwise only reached
by the few tests that nest deeper than that.

JSON_NO_THREAD_LOCAL switches the descent off, which sends every value
down that path. Running the whole test suite that way covers it with
every object type, string type, allocator, and base class the suite
already exercises. The new ci_test_no_thread_local target does that; the
macro had no build coverage at all before.

Copying a nested value also has to carry over what the element-wise copy
constructor would have copied: the parents that JSON_DIAGNOSTICS relies
on, and the positions that JSON_DIAGNOSTIC_POSITIONS reports. Both are
now checked on either side of the descent bound, for objects and arrays.
Neither was tested before, and dropping either one makes the new tests
fail.

Also quantify what JSON_NO_THREAD_LOCAL costs a copy instead of calling
it "measurably slower".

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/ubuntu.yml

@@ -100,7 +100,7 @@ jobs:
     container: ubuntu:focal
     strategy:
       matrix:
-        target: [ci_cmake_flags, ci_test_diagnostics, ci_test_diagnostic_positions, ci_test_noexceptions, ci_test_noimplicitconversions, ci_test_legacycomparison, ci_test_noglobaludls]
+        target: [ci_cmake_flags, ci_test_diagnostics, ci_test_diagnostic_positions, ci_test_noexceptions, ci_test_noimplicitconversions, ci_test_legacycomparison, ci_test_noglobaludls, ci_test_no_thread_local]
     steps:
       - name: Install build-essential
         run: apt-get update ; apt-get install -y build-essential unzip wget git libssl-dev