MB-46360: Fix hang in ManifestTest/badNames under AArch64

The for loop in this test never exits when compiled under
AArch64. Strictly speaking this is correct behaviour; given 'char' is
actually unsigned by default on AArch64. Fix by using explicitly
signed char type.

Change-Id: I146ab760cfde25ab33a79c653af8cb9502ddc47c
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/153827
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@couchbase.com>

engines/ep/tests/module_tests/collections/manifest_test.cc

@@ -649,7 +649,7 @@ TEST(ManifestTest, toJson) {
 #endif // !defined(__clang_major__) || __clang_major__ > 7
 
 TEST(ManifestTest, badNames) {
-    for (char c = 127; c >= 0; c--) {
+    for (int8_t c = 127; c >= 0; c--) {
         std::string name(1, c);
         CollectionsManifest cm({name, 8});