Make testing spack commands simpler (#4868)

Adds SpackCommand class allowing Spack commands to be easily in Python

Example usage:

    from spack.main import SpackCommand
    info = SpackCommand('info')
    out, err = info('mpich')
    print(info.returncode)

This allows easier testing of Spack commands.

Also:
* Simplify command tests
* Simplify mocking in command tests.
* Simplify module command test
* Simplify python command test
* Simplify uninstall command test
* Simplify url command test
* SpackCommand uses more compatible output redirection

var/spack/repos/builtin.mock/packages/a/package.py

@@ -26,7 +26,7 @@
 
 
 class A(AutotoolsPackage):
-    """Simple package with no dependencies"""
+    """Simple package with one optional dependency"""
 
     homepage = "http://www.example.com"
     url      = "http://www.example.com/a-1.0.tar.gz"

var/spack/repos/builtin.mock/packages/libdwarf/package.py

@@ -41,4 +41,4 @@ class Libdwarf(Package):
     depends_on("libelf")
 
     def install(self, spec, prefix):
-        pass
+        touch(prefix.libdwarf)

var/spack/repos/builtin.mock/packages/libelf/package.py

@@ -34,11 +34,4 @@ class Libelf(Package):
     version('0.8.10', '9db4d36c283d9790d8fa7df1f4d7b4d9')
 
     def install(self, spec, prefix):
-        configure("--prefix=%s" % prefix,
-                  "--enable-shared",
-                  "--disable-dependency-tracking",
-                  "--disable-debug")
-        make()
-
-        # The mkdir commands in libelf's intsall can fail in parallel
-        make("install", parallel=False)
+        touch(prefix.libelf)