Merge branch 'bug-5137b43870--clock-test-singleproc'

fixes [5137b4387019d0e1](https://core.tcl-lang.org/tcl/info/5137b4387019d0e1): ensure cache of base is correct for :localtime (system TZ) if TZ-env changing, especially if there is no registry;
clock.test: several tests rewritten as locale, timezone, environment (TZ, TCL_TZ) and also registry independent, also improved several setups/cleanups and usage of test-own registry - there are also no test failures by `-singleproc 1` anymore in clock.test

Author: sebres

generic/tclClock.c

@@ -1035,7 +1035,11 @@ ClockConfigureObjCmd(
 		    Tcl_SetObjRef(dataPtr->systemTimeZone, objv[i]);
 		    Tcl_UnsetObjRef(dataPtr->systemSetupTZData);
 		}
-		dataPtr->lastTZEpoch = lastTZEpoch;
+		if (dataPtr->lastTZEpoch != lastTZEpoch) {
+		    dataPtr->lastTZEpoch = lastTZEpoch;
+		    /* TZ epoch changed - invalidate base-cache */
+		    Tcl_UnsetObjRef(dataPtr->lastBase.timezoneObj);
+		}
 	    }
 	    if (i+1 >= objc && dataPtr->systemTimeZone != NULL
 			 && dataPtr->lastTZEpoch == lastTZEpoch) {

lib/clock.tcl

@@ -668,14 +668,17 @@ proc ::tcl::clock::EnterLocale { locale } {
 
 #----------------------------------------------------------------------
 #
+# _registryExists --
 # _hasRegistry --
 #
-#	Helper that checks whether registry module is available (Windows only)
+#	Helpers that checks whether registry module is available (Windows only)
 #	and loads it on demand.
 #
+# Side effects:
+#	_hasRegistry does it only once, and hereafter simply returns 1 or 0.
+#
 #----------------------------------------------------------------------
-proc ::tcl::clock::_hasRegistry {} {
-    set res 0
+proc ::tcl::clock::_registryExists {} {
     if { $::tcl_platform(platform) eq {windows} } {
 	if { [catch { package require registry 1.1 }] } {
 	    # try to load registry directly from root (if uninstalled / development env):
@@ -687,9 +690,13 @@ proc ::tcl::clock::_hasRegistry {} {
 	    }}
 	}
 	if { [namespace which -command ::registry] ne "" } {
-	    set res 1
+	    return 1
 	}
     }
+    return 0
+}
+proc ::tcl::clock::_hasRegistry {} {
+    set res [_registryExists]
     proc ::tcl::clock::_hasRegistry {} [list return $res]
     return $res
 }