bisync: fix time.Local data race on tests - fixes #8272

Before this change, the bisync tests were directly setting the time.Local
variable to UTC.

The reason for overriding the time zone on the tests is to make them
deterministic regardless of where in the world the user happens to be. There are
some goldenized strings which have the time zone hard-coded and would result in a
miscompare failure outside of that time zone.

However, mutating the time.Local variable is not the right way to do this, as OP
correctly pointed out on #8272.

Setting the TZ environment variable from within the code was also not an ideal
solution because, while it worked on unix, it did not work on Windows. See
fbac94a799/src/time/zoneinfo.go (L79-L80)

This change fixes the issue by defining a new bisync.LogTZ setting for use when
printing timestamps in /cmd/bisync/resolve.go. We override this on the tests
instead of time.Local.

cmd/bisync/bisync_test.go

@@ -226,6 +226,7 @@ var color = bisync.Color
 
 // TestMain drives the tests
 func TestMain(m *testing.M) {
+	bisync.LogTZ = time.UTC
 	fstest.TestMain(m)
 }
 
@@ -277,7 +278,6 @@ func testBisync(t *testing.T, path1, path2 string) {
 		ci.RefreshTimes = true
 	}
 	bisync.Colors = true
-	time.Local = bisync.TZ
 	ci.FsCacheExpireDuration = fs.Duration(5 * time.Hour)
 
 	baseDir, err := os.Getwd()