Add DMI test fixtures from linuxhw/DMI and expand placeholder detection

Adds board and memory parser test fixtures based on real dmidecode output
from Dell PowerEdge R740xd, HPE ProLiant DL380 Gen10, and Supermicro
SYS-6028R-WTR sourced from the linuxhw/DMI dataset. Extends cleanDMIValue
with four additional vendor placeholder strings found in the dataset:
"0123456789", "1234567890", "NOT AVAILABLE", and "TO BE FILLED BY O.E.M"
(without trailing dot). Adds memory_test.go covering mixed populated/empty
DIMM slots and both GB and MB size formats.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/collector/board.go

@@ -174,15 +174,19 @@ func cleanDMIValue(v string) string {
 	upper := strings.ToUpper(v)
 	placeholders := []string{
 		"TO BE FILLED BY O.E.M.",
+		"TO BE FILLED BY O.E.M",
 		"NOT SPECIFIED",
 		"NOT SETTABLE",
 		"NOT PRESENT",
+		"NOT AVAILABLE",
 		"UNKNOWN",
 		"N/A",
 		"NONE",
 		"NULL",
 		"DEFAULT STRING",
 		"0",
+		"0123456789",
+		"1234567890",
 	}
 	for _, p := range placeholders {
 		if upper == p {