tests: posix: common: free resource in after() for timer suite

Create a separate ZTEST_SUITE() to properly manage resources
that are shared between test cases. Add some assertions where
they were missing.

Additionally, we don't need verbose printing in this testsuite,
so change `printk()` to `LOG_DBG()`

Signed-off-by: Christopher Friedt <cfriedt@meta.com>