tests: Optional fallback subprojects can fail

Tests https://github.com/mesonbuild/meson/issues/975

test cases/common/95 dep fallback/meson.build

@@ -5,6 +5,7 @@ if not bob.found()
     error('Bob is actually needed')
 endif
 jimmy = dependency('jimmylib', fallback : ['jimmylib', 'jimmy_dep'], required: false)
+dummy = dependency('dummylib', fallback : ['dummylib', 'dummy_dep'], required: false)
 
 gensrc_py = find_program('gensrc.py')
 gensrc = custom_target('gensrc.c',

test cases/common/95 dep fallback/subprojects/dummylib/meson.build

@@ -0,0 +1,4 @@
+project('dummylib', 'c')
+
+dummy_dep = declare_dependency()
+error('this subproject fails to configure')