packer_test: rename core test run function

The function's name was the same as the plugins test suite runner/init
function, making it impossible to differentiate between the two when
looking at the logs, or when attempting to filter which tests to run
using the suite function's name.

packer_test/core_tests/suite_test.go

@@ -11,7 +11,7 @@ type PackerCoreTestSuite struct {
 	*lib.PackerTestSuite
 }
 
-func Test_PackerPluginSuite(t *testing.T) {
+func Test_PackerCoreSuite(t *testing.T) {
 	baseSuite, cleanup := lib.InitBaseSuite(t)
 	defer cleanup()