Add tests for the "%#04x" pattern

Verifies that values other than 0 are prepended with "0x", while 0 is printed as "0000".

PiperOrigin-RevId: 549108651
Change-Id: Ib0f87d42066c46f3a5d4b05ca5c9d7ac17be9b66

absl/strings/str_format_test.cc

@@ -638,6 +638,8 @@ TEST(StrFormat, BehavesAsDocumented) {
   EXPECT_EQ(StrFormat("%#o", 10), "012");
   EXPECT_EQ(StrFormat("%#x", 15), "0xf");
   EXPECT_EQ(StrFormat("%04d", 8), "0008");
+  EXPECT_EQ(StrFormat("%#04x", 0), "0000");
+  EXPECT_EQ(StrFormat("%#04x", 1), "0x01");
   // Posix positional substitution.
   EXPECT_EQ(absl::StrFormat("%2$s, %3$s, %1$s!", "vici", "veni", "vidi"),
             "veni, vidi, vici!");