Update test_excel

[junkmd]

In #298, environment-specific-tests are skipped.

But there are def setUpModule;raise SkipTest(... in some modules.
In this way, skipped testcase names are not displayed even if using -v option.

With this change, in test_excel, skipped testcase name and reasons are displayed in console.

for example:

# with non-excel-installed environment
test (test_excel.Test_EarlyBind) ... skipped 'This depends on Excel.'
test (test_excel.Test_LateBind) ... skipped 'This depends on Excel.'

# with excel-installed environment
test (test_excel.Test_EarlyBind) ... skipped 'There is difference of `Range.Value` behavior between Python >= 3.8.x and Python <= 3.7.x.'
test (test_excel.Test_LateBind) ... ok

See #212 (comment) for details of @unittest.skip("There is difference of ...