Make tests use mock compiler configuration.

- makes sure tests don't fail on systems that don't have some compilers (e.g. clang).
- more control over specific test cases for compilers.

var/spack/mock_configs/site_spackconfig

@@ -0,0 +1,12 @@
+[compiler "gcc@4.5.0"]
+    cc  = /path/to/gcc
+    cxx = /path/to/g++
+    f77 = /path/to/gfortran
+    fc  = /path/to/gfortran
+
+[compiler "clang@3.3"]
+    cc  = /path/to/clang
+    cxx = /path/to/clang++
+    f77 = None
+    fc  = None
+