[Tests] Properly link Linux tests

#46 is missing
this change to the compiler invocation, which allows tests to run
on Linux even when `--library-install-path` and
`--module-install-path` are not specified.

Author: modocache

Tests/Functional/lit.cfg

@@ -33,6 +33,8 @@ swift_exec = [
     _getenv('SWIFT_EXEC'),
     '-Xlinker', '-rpath',
     '-Xlinker', built_products_dir,
+    '-L', built_products_dir,
+    '-I', built_products_dir,
 ]
 
 if platform.system() == 'Darwin':
@@ -48,8 +50,6 @@ if platform.system() == 'Darwin':
     swift_exec.extend([
         '-sdk', sdk_root,
         '-target', target,
-        '-L', built_products_dir,
-        '-I', built_products_dir,
         '-F', built_products_dir,
     ])