fastrpc_test: default to /usr/local/bin and set LD_LIBRARY_PATH for local libs

[smuppand]

What changed

• Default binary location: --bin-dir now defaults to /usr/local/bin (was /usr/bin).
• Library path: export LD_LIBRARY_PATH=/usr/local/lib/fastrpc_test before launching the test so bundled libs are found.
• Help text & logs: updated notes and messages to reflect the new default path.
• Safety check: existing /bin guard retained; message updated to suggest /usr/local/bin.

Why

• fastrpc_test and its libs are being installed under /usr/local; this makes the runner work out-of-the-box without extra flags.
• Ensures the test picks up its companion libs from /usr/local/lib/fastrpc_test.

Backward compatibility

• Users can still override with --bin-dir (e.g., /usr/bin).
• ALLOW_BIN_FASTRPC=1 still permits /bin/fastrpc_test.
• Default invocation (no flags)
• export LD_LIBRARY_PATH=/usr/local/lib/fastrpc_test
• (cd /usr/local/bin && ./fastrpc_test -d -t linux -U 0)

[abbajaj806]

Looks good to me