Fix core dump when invalid MAC is configured

When configuring an invalid MAC, a runtime error is thrown
with the existing implementation, which causes a core dump
and phosphor-networkd does not respond when further requests
are made.

This commit avoids the core dump that's caused.

Signed-off-by: Asmitha Karunanithi <asmitk01@in.ibm.com>
Change-Id: I5cb5a25e91a685e1bb2f4bb80d3d8f8c483f56a1
diff --git a/test/test_util.cpp b/test/test_util.cpp
index a6605dd..e3a1fba 100644
--- a/test/test_util.cpp
+++ b/test/test_util.cpp
@@ -219,9 +219,9 @@
 
 TEST(MacFromString, Bad)
 {
-    EXPECT_THROW(fromString("0x:00:00:00:00:00"), std::runtime_error);
-    EXPECT_THROW(fromString("00:00:00:00:00"), std::runtime_error);
-    EXPECT_THROW(fromString(""), std::runtime_error);
+    EXPECT_THROW(fromString("0x:00:00:00:00:00"), std::invalid_argument);
+    EXPECT_THROW(fromString("00:00:00:00:00"), std::invalid_argument);
+    EXPECT_THROW(fromString(""), std::invalid_argument);
 }
 
 TEST(MacFromString, Valid)