tests: add multivar read test

tests-clar/config/multivar.c

+#include "clar_libgit2.h"
+
+static int mv_read_cb(const char *name, const char *GIT_UNUSED(value), void *data)
+{
+	int *n = (int *) data;
+
+	if (!strcmp(name, "remote.fancy.fetch"))
+		(*n)++;
+
+	return 0;
+}
+
+void test_config_multivar__foreach(void)
+{
+	git_config *cfg;
+	int n = 0;
+
+	cl_git_pass(git_config_open_ondisk(&cfg, cl_fixture("config/config11")));
+
+	cl_git_pass(git_config_foreach(cfg, mv_read_cb, &n));
+	cl_assert(n == 2);
+
+	git_config_free(cfg);
+}

tests/resources/config/config11

+[remote "fancy"]
+    fetch = git://github.com/libgit2/libgit2
+    fetch = git://git.example.com/libgit2