sh.py should not try to execute a folder

Problem: sh attempts to execute folders because which(1) fails
         to determine that although folders may have execute-bit
         set, they are not executable by execv(3)-family calls

Symptom: such call is blocking, never returns (!)

Solution: check the target is a file (following symlinks).

Details: The internal which(1) clone will attempt to execute any
         folder matching the Command issued. So, given the PATH of:

              /usr/opt/bin/gcc   <- directory
              /usr/bin/gcc       <- executable file

         sh would attempt to execute the folder, /usr/opt/bin/gcc.

[ As executing a folder would cause sh to block indefinitely
  (an unhandled OSError is thrown on exec), the test adds a ._path
  value assertion first, which prevents this test from hanging when
  the fix is un-applied.  ]