Add test that missing requires cause a config failure

tools/test/config/lib_requires_missing_lib/lib1/mbed_lib.json

@@ -0,0 +1,7 @@
+{
+    "name": "lib1",
+    "requires": ["lib2"],
+    "config": {
+        "test": "BAD"
+    }
+}

tools/test/config/lib_requires_missing_lib/lib2/mbed_lib.json

@@ -0,0 +1,9 @@
+{
+    "name": "lib2",
+    "requires": ["lib3"],
+    "config": {
+        "test": {
+            "value": "BAD"
+        }
+    }
+}

tools/test/config/lib_requires_missing_lib/mbed_app.json

@@ -0,0 +1,9 @@
+{
+    "requires" : ["lib1"],
+    "target_overrides": {
+        "test_target": {
+            "lib2.test": "GOOD",
+            "lib1.test": "GOOD"
+        }
+    }
+}

tools/test/config/lib_requires_missing_lib/targets.json

@@ -0,0 +1,9 @@
+{
+    "test_target": {
+        "supported_toolchains": ["GCC_ARM"],
+        "core": "Cortex-M0",
+        "extra_labels": [],
+        "features": [],
+        "default_lib": "std"
+    }
+}

tools/test/config/lib_requires_missing_lib/test_data.json

@@ -0,0 +1,5 @@
+{
+    "test_target": {
+    	"exception_msg": "'lib2' requires 'lib3' which is not present"
+    }
+}