shtests: -l: make sure radix point is '.'

Using the bin/shtests -l/--locale option to run the regression
tests in your own locale broke the tests if you're in a locale that
uses ',' as the radix point, like my nl_NL.UTF-8, unless
LC_NUMERIC=C was exported manually. Let's automate that fix.

src/cmd/ksh93/tests/shtests: --locale:
- If LC_ALL was set, copy it to LANG and unset all LC_* vars.
  This allows overriding the radix point with LC_NUMERIC if needed.
- If '1.0' is not a valid shell arithmetic expression, export
  LC_NUMERIC=C to fix it.

src/cmd/ksh93/tests/shtests

@@ -10,7 +10,7 @@ valgrindflags='--xml=yes --log-file=/dev/null --track-origins=yes --read-var-inf
 
 USAGE=$'
 [-s8?
-@(#)$Id: shtests (AT&T Research/ksh93) 2020-07-04 $
+@(#)$Id: shtests (AT&T Research/ksh93) 2020-08-11 $
 ]
 '$USAGE_LICENSE$'
 [+NAME?shtests - ksh regression test harness]
@@ -33,9 +33,9 @@ USAGE=$'
 [d:debug?Enable \bshtests\b execution trace.]
 [k:keep?Keep temporary files after test run; shtests will report the location.]
 [l:locale?Disable \b--utf8\b and run the \b--posix\b and \b--compile\b
-    tests, if enabled, in the locale of the caller. This may cause invalid
+    tests, if enabled, in the locale of the caller. However, for locales
-    regressions, especially for locales where \b.\b is not the radix
+    where \b.\b is not the radix point, \bLC_NUMERIC\b is set to \bC\b
-    point.]
+    to avoid invalid regressions.]
 [p:posix?Run the test scripts in the posix/C locale.]
 [t!:time?Include the current date/time in the test identification
     messages.]
@@ -234,6 +234,10 @@ if	(( locale ))
 then	utf8=0
 	if	[[ $LC_ALL ]]
 	then	export LANG=$LC_ALL
+		unset ${!LC_*}
+	fi
+	if	! let 1.0 2>/dev/null
+	then	export LC_NUMERIC=C
 	fi
 else	unset LANG LC_ALL
 	export LC_NUMERIC=C