remove unecessary inexistant block panic + add test

the hcl library will error for us

hcl2template/parser.go

@@ -127,8 +127,6 @@ func (p *Parser) parseFile(f *hcl.File, cfg *PackerConfig) hcl.Diagnostics {
 			}
 			cfg.Builds = append(cfg.Builds, build)
 
-		default:
-			panic(fmt.Sprintf("unexpected block type %q", block.Type)) // TODO(azr): err
 		}
 	}
 

hcl2template/testdata/unknown/block_type.pkr.hcl

@@ -0,0 +1,3 @@
+
+potato {
+}

hcl2template/types.packer_config_test.go

@@ -90,6 +90,13 @@ func TestParser_complete(t *testing.T) {
 			getBuildsWantBuilds:    []packer.Build{},
 			getBuildsWantDiags:     true,
 		},
+		{name: "unknown block type",
+			parser:                 defaultParser,
+			args:                   parseTestArgs{"testdata/unknown"},
+			parseWantCfg:           &PackerConfig{},
+			parseWantDiags:         true,
+			parseWantDiagHasErrors: true,
+		},
 	}
 	testParse(t, tests)
 }