Fix validation of target_exe blob name

[ranweiler]

Summary

Our validation of target_exe blob names spuriously failed when the target was in a nested (virtual) directory. Fix validation to support this case, and check additional Azure blob storage naming criteria. Also, add unit tests.

Closes #1362.

Explanation

A bug-witnessing target_exe value is bin/fuzz.exe. This should be accepted.

But consider:

>>> import ntpath, posixpath
>>> target_exe = 'bin/fuzz.exe'
>>> posixpath.relpath(target_exe)
'bin/fuzz.exe'
>>> ntpath.relpath(target_exe)
'bin\\fuzz.exe'

Our validation required that each relpath() invocation above is equivalent to the identity function. But the output of the last relpath() call differs from the input.

Testing

• Can create a job whose target_exe is in the root of setup_dir
• Can create a job whose target_exe is in a nested directory, like {setup_dir}/bin/fuzz.exe (from Linux, on Linux pool)
• Can create a job whose target_exe is in a nested directory, like {setup_dir}\\bin\\fuzz.exe (from Windows, on Windows pool)

The last test case ensures [1] that the CLI normalizes relativized paths as POSIX-style, which is consistent with the Blob Storage Service's use of forward slash (/) as the virtual directory delimiter.

[1] It does not. This test passes, because the Windows-style paths still (happen to) work on Windows VMs. Linux jobs created from Windows with this path style fail at task runtime. Captured in #1373.