Fix a test failure in TestSuiteTest:test_fromName_with_relative_dirname_on_path on R2012a

[arthurward]

PR #16, the fix for #14, introduced a test failure on R2012a (Windows, 64-bit). "TestSuiteTest:test_fromName_with_relative_dirname_on_path" fails with the message below because matlab.system.CoreBlockSystem contains an error:

The specified super-class 'matlab.system.BlockCore' contains a parse error or cannot be found on MATLAB's search path, possibly shadowed by another file with the same name.

I modified the test to use its own directory on the path, rather than assuming anything about MATLAB's internal packages. I verified that this version of the test fails in R2012a and R2017a when the fix for TestSuite.fromName is backed out.

[arthurward]

I also discovered that the workaround from #16 fails when a directory is shadowing a package of the same name. The second commit here adds a test case for this and fixes TestSuite.fromName by replacing isdir() with fileattrib().