Use \" instead of ' for exiv2 command string

The original failed on some Windows test runs.

tests/bugfixes/github/test_pr_1999.py

@@ -10,16 +10,16 @@ class TestXmpDateTimeSetting(metaclass=CaseMeta):
 
     infile = path("$data_path/issue_1998.xmp")
     commands = [
-        "$exiv2 -M'set Xmp.xmp.CreateDate XmpText 2021-02-03T12:00:00+01:00' $infile",
+        "$exiv2 -M\"set Xmp.xmp.CreateDate XmpText 2021-02-03T12:00:00+01:00\" $infile",
         "$exiv2 -K Xmp.xmp.CreateDate $infile",
-        "$exiv2 -M'set Xmp.xmp.CreateDate XmpText 2021-02-03T12:34:56+02:00' $infile",
+        "$exiv2 -M\"set Xmp.xmp.CreateDate XmpText 2021-02-03T12:34:56+02:00\" $infile",
         "$exiv2 -K Xmp.xmp.CreateDate $infile",
         ]
     stdout = [
-        """""",
+        "",
         """Xmp.xmp.CreateDate                           XmpText    25  2021-02-03T12:00:00+01:00
 """,
-        """""",
+        "",
         """Xmp.xmp.CreateDate                           XmpText    25  2021-02-03T12:34:56+02:00
 """,
               ]