fix: test: Register orphaned diff and skr unit tests in meson build

Both test files existed on disk but were never added to the meson test
list when the build system switched from autoconf.

skr_test.c also had a spurious #include <dns/tls.h> for a header that
never existed in this repo -- no symbols from it were used. Removing
the include is the only fix needed; the test itself is correct and
passes.

Assisted-by: Claude:claude-opus-4-7
Merge branch 'mnowak/fix-orphaned-unit-tests' into 'main'

See merge request isc-projects/bind9!12091