fix(test): make the client_mss config assertions actually test something

[rvalitov]

Fixes #144

Problem

generate_telemt_config() takes a destination path and writes the TOML there — it does not print it. This test captured its stdout instead:

cfg=$(generate_telemt_config)
if echo "$cfg" | grep -q 'client_mss = "tspu"'; then ...

cfg is therefore always empty, so both config assertions were decided by that emptiness rather than by the generated config:

• "client_mss omitted when off" passed vacuously. grep found nothing in an empty string, so it reported absent regardless of what the product did — including when client_mss was emitted unconditionally.
• "client_mss emitted when set to tspu" could never pass, on any platform.

Demonstrated

Against a product mutated to always emit client_mss = "tspu", this test produced byte-identical output to the unmutated product:

8 tests, 1 failures     <- mutated product
8 tests, 1 failures     <- unmutated product

It had no signal at all.

Fix

Write to a temp destination and grep that file. An explicit "config is written" assertion is included so that if generation ever fails, the test fails loudly instead of passing vacuously again.

Testing

• Passes on Debian 12, Ubuntu 22.04/24.04, Alpine 3.20 and Fedora 41.
• Now fails correctly under both mutations — emission removed, and emission made unconditional — where the old test caught neither.

Found by running the existing suite across a distro matrix; this was the one failure common to every platform, which is what pointed at the test rather than at the product.