cleanup: Map test files according to structure in src

For example cmd_account() is located in src/command/cmd_funcs.c.
The unit test was located in tests/unittests/test_cmd_account.c.
Let's move it to a subdirectory like tests/unittests/command/test_cmd_account.c
to correpsond to the same structure.

tests/unittests/plugins/test_callbacks.c

@@ -0,0 +1,57 @@
+#include "prof_cmocka.h"
+#include <stdlib.h>
+#include <string.h>
+#include <glib.h>
+
+#include "plugins/callbacks.h"
+#include "plugins/plugins.h"
+
+void
+returns_no_commands(void** state)
+{
+    plugins_init();
+    GList* commands = plugins_get_command_names();
+
+    assert_true(commands == NULL);
+}
+
+void
+returns_commands(void** state)
+{
+    plugins_init();
+    PluginCommand* command1 = g_new0(PluginCommand, 1);
+    command1->command_name = g_strdup("command1");
+    callbacks_add_command("plugin1", command1);
+
+    PluginCommand* command2 = g_new0(PluginCommand, 1);
+    command2->command_name = g_strdup("command2");
+    callbacks_add_command("plugin1", command2);
+
+    PluginCommand* command3 = g_new0(PluginCommand, 1);
+    command3->command_name = g_strdup("command3");
+    callbacks_add_command("plugin2", command3);
+
+    GList* names = plugins_get_command_names();
+    assert_true(g_list_length(names) == 3);
+
+    gboolean foundCommand1 = FALSE;
+    gboolean foundCommand2 = FALSE;
+    gboolean foundCommand3 = FALSE;
+    GList* curr = names;
+    while (curr) {
+        if (g_strcmp0(curr->data, "command1") == 0) {
+            foundCommand1 = TRUE;
+        }
+        if (g_strcmp0(curr->data, "command2") == 0) {
+            foundCommand2 = TRUE;
+        }
+        if (g_strcmp0(curr->data, "command3") == 0) {
+            foundCommand3 = TRUE;
+        }
+        curr = g_list_next(curr);
+    }
+
+    assert_true(foundCommand1 && foundCommand2 && foundCommand3);
+
+    g_list_free(names);
+}