Fix/workaround compilation error on devtoolset-6

Smells like GCC bug [1], which is backed up by the fact that locally
GCC-10 and CLang-10 works fine.

Since the change is trivial (and, arguably, correct since there is
no real difference between tests code and actual code) seems fine to
fix.

Fix suggested by Jacques Lucke, thanks!

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56480

tests/gtests/blenlib/BLI_set_test.cc

@@ -9,7 +9,7 @@
 #include "BLI_vector.hh"
 #include "testing/testing.h"
 
-using namespace blender;
+namespace blender {
 
 TEST(set, DefaultConstructor)
 {
@@ -290,7 +290,7 @@ bool operator==(const Type2 &a, const Type1 &b)
   return a.value == b.value;
 }
 
-template<> struct blender::DefaultHash<Type1> {
+template<> struct DefaultHash<Type1> {
   uint32_t operator()(const Type1 &value) const
   {
     return value.value;
@@ -511,3 +511,5 @@ TEST(set, Benchmark)
  */
 
 #endif /* Benchmark */
+
+}  // namespace blender