Adapt NetBSD t_fmemopen test for picolibc

The test checks some NetBSD-specific behaviour:rejecting non-NUL
writes at the end of the buffer as well as assuming that opening in 'w'
mode always writes a NUL byte at the start of the buffer.
Also fix it for glibc so that we can run the native test.

Author: arichardson

test/meson.build

@@ -488,6 +488,10 @@ foreach target : targets
     plain_tests += 'malloc_stress'
   endif
 
+  if tinystdio
+    plain_tests += 't_fmemopen'
+  endif
+
   if (posix_io or not tinystdio) and tests_enable_posix_io
     plain_tests += ['posix-io']