Use test -d with directories, not test -f.

Context: https://github.com/xamarin/monodroid/pull/712#issuecomment-349762244

In attempting to bump the xamarin-android commit reference within
monodroid, monodroid's inter-API-level checks failed:

	<h1>### API BREAK BETWEEN v4.3 and v4.4</h1>
	...
	<h3>Type Changed: Android.Views.Animations.Transformation</h3>
	<p>Removed properties:</p>

	<pre>
		<span class='removed removed-property breaking' data-is-breaking>public static int TypeAlpha { get; set; }</span>
		<span class='removed removed-property breaking' data-is-breaking>public static int TypeBoth { get; set; }</span>
		<span class='removed removed-property breaking' data-is-breaking>public static int TypeIdentity { get; set; }</span>
		<span class='removed removed-property breaking' data-is-breaking>public static int TypeMatrix { get; set; }</span>
	</pre>

The question, then: Why didn't *xamarin-android* fail for the same
reason? Manually inspecting the resulting assemblies shows that the
same problem exists in the OSS Jenkins build artifacts for
`Mono.Android.dll` between v4.3 and v4.4, so why wasn't this caught by
the appropriate xamarin-android PR?

The reason? `make check-inter-api-level` was mis-using **test**(1):

	if [ ! -f "$(XA_FRAMEWORK_DIR)/$${_frameworks[$$i]}" ]

`$(XA_FRAMEWORK_DIR)/$${_frameworks[$$i]}` will be a *directory*, not
a file, and `test -f /` (or any other directory) will always fail.
Consequently, we *never* generated the `inter-apis/*/Mono.Android.xml`
files, preventing us from performing the inter-API-level checks.

Doh!

Fix our **test**(1) invocation so that it works, using `test -d`.

Author: jonpryor

Makefile

@@ -118,7 +118,8 @@ check-inter-api-level: -create-inter-api-infos
 	failed=0; \
 	_frameworks=($(STABLE_FRAMEWORKS)) ; \
 	for (( i = 1; i < $${#_frameworks[@]}; i++ )) ; do \
-		if [ ! -f "$(XA_FRAMEWORK_DIR)/$${_frameworks[$$i]}" ] ; then \
+		if [ ! -d "$(XA_FRAMEWORK_DIR)/$${_frameworks[$$i]}" ] ; then \
+			echo "# Framework directory '$(XA_FRAMEWORK_DIR)/$${_frameworks[$$i]}' doesn't exist. Skipping..."; \
 			continue ; \
 		fi; \
 		prev_framework=$$(expr $$i - 1); \