time: skip ZoneAbbr test in timezones with no abbreviation

The testZoneAbbr assumes that

  Parse(RFC1123, t1.Format(RFC1123))

will always succeed. This is not true because Format will fall back to
the numeric zone (ex. -07) for timezones with no abbreviation, but
Parse won't accept the numeric zone when the layout specifies 'MST'
(an abbreviation).

Skip the zone abbreviation test in timezones with no abbreviation.

Fixes #21183

Change-Id: If04691cc23ae1075d8a953733024e17f5a7646de
Reviewed-on: https://go-review.googlesource.com/52430
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/zoneinfo_windows_test.go

@@ -14,6 +14,14 @@ func testZoneAbbr(t *testing.T) {
 	t1 := Now()
 	// discard nsec
 	t1 = Date(t1.Year(), t1.Month(), t1.Day(), t1.Hour(), t1.Minute(), t1.Second(), 0, t1.Location())
+
+	// Skip the test if we're in a timezone with no abbreviation.
+	// Format will fallback to the numeric abbreviation, and
+	// Parse(RFC1123, ..) will fail (see Issue 21183).
+	if tz := t1.Format("MST"); tz[0] == '-' || tz[0] == '+' {
+		t.Skip("No zone abbreviation")
+	}
+
 	t2, err := Parse(RFC1123, t1.Format(RFC1123))
 	if err != nil {
 		t.Fatalf("Parse failed: %v", err)