Update meson dependency usage

test/meson.build

@@ -7,8 +7,7 @@ test_sources = files([
       'test_extractions_without_fast_float.cpp',
       ])
 
-doctest_proj = subproject('doctest')
-doctest_dep = doctest_proj.get_variable('doctest_dep')
+doctest_dep = dependency('doctest')
 
 test_exe = executable(
   'test_ssp',

test/test_helpers.hpp

@@ -1,12 +1,7 @@
 #pragma once
 #include <cstdlib>
 #include <cstring>
-
-#ifdef CMAKE_GITHUB_CI
 #include <doctest/doctest.h>
-#else
-#include <doctest.h>
-#endif
 
 struct buffer {
     char* data_{nullptr};

test/test_main.cpp

@@ -1,2 +1,2 @@
 #define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
-#include <doctest.h>
+#include <doctest/doctest.h>