test(plugin): add tests for pkg/rand (#7320)

Add test suite covering thread-safe random number generator with
tests for:

- Constructor with various seed values (positive, zero, negative)
- Deterministic behavior verification with same seeds
- Permutation generation and validation
- Concurrent access safety with multiple goroutines
- Mixed operations under concurrent load

Also clarify package documentation to explicitly state this is
for load balancing and server selection, not cryptographic use.
The math/rand usage is intentional for performance in non-security
contexts like upstream server selection and DNS record shuffling.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/pkg/rand/rand.go

@@ -1,4 +1,7 @@
 // Package rand is used for concurrency safe random number generator.
+// This package provides a thread-safe wrapper around math/rand for use in
+// load balancing and server selection. It is NOT suitable for cryptographic
+// purposes and should not be used for security-sensitive operations.
 package rand
 
 import (