Combining subcases and threads can put doctest into an infinite loop

[patstew]

TEST_CASE("crash") {
    SUBCASE("sub") { CHECK(true); }
    std::jthread([] { REQUIRE(false); });
};

This test case causes doctest to try and print out its subcase stack 2^64-1 times. The same thing happens with any exception, segfault etc in the sub thread, not just REQUIRE. The problem is that reportFault calls subcase_end according to the max stack depth, not the current stack location, so currentSubcaseLevel gets decremented below 0. Then the logTestStart attempts to print that many names from the subcase stack, with sad results.