Add sem_try_acquire(). Fixes #846 (#856)

Co-authored-by: Geoff Simmons <geoff@uplex.de>

test/pico_sem_test/CMakeLists.txt

@@ -0,0 +1,4 @@
+add_executable(pico_sem_test pico_sem_test.c)
+
+target_link_libraries(pico_sem_test PRIVATE pico_test pico_sync)
+pico_add_extra_outputs(pico_sem_test)

test/pico_sem_test/pico_sem_test.c

@@ -0,0 +1,29 @@
+/**
+ * Copyright (c) 2022 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <stdio.h>
+
+#include "pico/sem.h"
+#include "pico/test.h"
+#include "pico/stdio.h"
+
+PICOTEST_MODULE_NAME("SEM", "semaphore test");
+
+int main() {
+    semaphore_t sem;
+
+    stdio_init_all();
+    sem_init(&sem, 1, 1);
+
+    PICOTEST_START();
+
+    PICOTEST_START_SECTION("sem_try_acquire");
+        PICOTEST_CHECK(sem_try_acquire(&sem), "available permit not acquired");
+        PICOTEST_CHECK(!sem_try_acquire(&sem), "success with no permits");
+    PICOTEST_END_SECTION();
+
+    PICOTEST_END_TEST();
+}