Создан сам тест

histogram_internal.h

@@ -1,6 +1,6 @@
 #ifndef HISTOGRAM_INTERNAL_H_INCLUDED
 #define HISTOGRAM_INTERNAL_H_INCLUDED
 #include <vector>
-void find_minmax( const std::vector<double>& numbers, double& minN, double& maxN)
+void find_minmax( const std::vector<double>& numbers, double& minN, double& maxN);
 
 #endif // HISTOGRAM_INTERNAL_H_INCLUDED

unittest.cpp

@@ -0,0 +1,13 @@
+#define DOCTEST_CONFIG_NO_MULTITHREADING
+#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
+#include "doctest.h"
+#include "histogram_internal.h"
+#include "histogram.h"
+
+TEST_CASE("distinct positive numbers") {
+    double minN = 0;
+    double maxN = 0;
+    find_minmax({1, 2}, minN, maxN);
+    CHECK(minN == 1);
+    CHECK(maxN == 2);
+}