simple test for exporting variables in static libs

test/standalone/static_c_lib/foo.c

@@ -2,3 +2,5 @@
 uint32_t add(uint32_t a, uint32_t b) {
     return a + b;
 }
+
+uint32_t foo = 12345;

test/standalone/static_c_lib/foo.h

@@ -1,2 +1,3 @@
 #include <stdint.h>
 uint32_t add(uint32_t a, uint32_t b);
+extern uint32_t foo;

test/standalone/static_c_lib/foo.zig

@@ -6,3 +6,7 @@ test "C add" {
     const result = c.add(1, 2);
     expect(result == 3);
 }
+
+test "C extern variable" {
+    expect(c.foo == 12345);
+}