add tests for Mutex (#925)

* std/lib: add tests for Mutex

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

* lib/collections: add missing import

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

* std/collections: add RingBuffer

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

---------

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

test/unit/stdlib/collections/ringbuffer.c3

@@ -0,0 +1,26 @@
+module ringbuffer_test @test;
+import collections::ringbuffer;
+import std::io;
+
+def Buffer = RingBuffer(<char, 4>);
+
+fn void putc_getc()
+{
+    Buffer rb;
+    rb.init();
+    rb.putc(1);
+    rb.putc(2);
+    rb.putc(3);
+    rb.putc(4);
+
+    assert(rb.getc(0) == 1);
+    assert(rb.getc(1) == 2);
+    assert(rb.getc(2) == 3);
+    assert(rb.getc(3) == 4);
+
+    rb.putc(5);
+    assert(rb.getc(0) == 2);
+    assert(rb.getc(1) == 3);
+	assert(rb.getc(2) == 4);
+	assert(rb.getc(3) == 5);
+}