Add a test where a single exe has both C and C++ sources.

test cases/7 mixed/builder.txt

@@ -0,0 +1,3 @@
+project('mixed C and C++', 'c', 'c++')
+exe = executable('prog', 'main.cc', 'func.c')
+add_test('mixtest', exe)

test cases/7 mixed/func.c

@@ -0,0 +1,4 @@
+int func() {
+    int class = 0;
+    return class;
+}

test cases/7 mixed/main.cc

@@ -0,0 +1,5 @@
+extern "C" int func();
+
+int main(int argc, char **argv) {
+    return func();
+}