Print subcase stack upon a subcase entry

This is largely a hack to give us at least *something* for data driven
testing (doctest#215). In that pattern, it's common to structure code in a way
that first "generates the data", and only then executes the actual test.
It can look like this:

  TEST_CASE("parsing stuff") {
    std::string input;
    Something expected;

    DOCTEST_SUBCASE("single-line") {
      DOCTEST_SUBCASE("empty") {}
      DOCTEST_SUBCASE("trivial") {
        input = "create foo";
	expected = {OP_CREATE, {"foo",}};
      }
      DOCTEST_SUBCASE("two args") {
        input = "create blah bar";
	expected = {OP_CREATE, {"blah", "bar",}};
      }
    }
    DOCTEST_SUBCASE("multi-line") {
      DOCTEST_SUBCASE("trailing whitespace") {
        input = "create foo\n\n";
	expected = {OP_CREATE, {"foo,}};
      }
      DOCTEST_SUBCASE("one word per line, two words") {
        input = "create\nfoo";
	expected = {OP_CREATE, {"foo",}};
      }
    }

    REQUIRE(parse(input) == expected);
  }

The important part is that once we're executing actual tests, the
subsections will have been exited already, so the usual ways of showing
the subcase stack can be no longer applied.

In Catch2, there was a patch which tried to track the deepest subcase
stack so far. Let's try to implement something very simple, just
printing the current state of the stack once it is entered. It does not
tie into any error handling, so the output will be always there, and it
also probably does not support any fancy filtering when skipping
subcases. However, it's something which at least shows what is going on
when an error gets reported.

Cc: doctest#215
Cc: doctest#125

doctest/doctest.h

@@ -5268,6 +5268,12 @@ namespace {
             std::lock_guard<std::mutex> lock(mutex);
             subcasesStack.push_back(subc);
             hasLoggedCurrentTestStart = false;
+
+            s << Color::Cyan << "[doctest] SUBCASE" << Color::None << "\n";
+            for(auto& curr : subcasesStack)
+                if(curr.m_name[0] != '\0')
+                    s << " - " << Color::LightGrey << curr.m_name << Color::None << " (" << curr.m_file << ":" << curr.m_line << ")\n";
+            s << "\n";
         }
 
         void subcase_end() override {

doctest/parts/doctest.cpp

@@ -2656,6 +2656,12 @@ namespace {
             std::lock_guard<std::mutex> lock(mutex);
             subcasesStack.push_back(subc);
             hasLoggedCurrentTestStart = false;
+
+            s << Color::Cyan << "[doctest] SUBCASE" << Color::None << "\n";
+            for(auto& curr : subcasesStack)
+                if(curr.m_name[0] != '\0')
+                    s << " - " << Color::LightGrey << curr.m_name << Color::None << " (" << curr.m_file << ":" << curr.m_line << ")\n";
+            s << "\n";
         }
 
         void subcase_end() override {