Update test to expect flit-core installation

The InstallPip() function now installs flit-core alongside pip,
so the unit test mock needs to expect the flit-core call.

Author: ivanovac

src/python/supply/supply_test.go

@@ -154,6 +154,7 @@ var _ = Describe("Supply", func() {
 
 			It("installs latest from manifest", func() {
 				mockInstaller.EXPECT().InstallOnlyVersion("pip", "/tmp/pip")
+				mockInstaller.EXPECT().InstallOnlyVersion("flit-core", "/tmp/pip")
 				mockCommand.EXPECT().Execute(buildDir, gomock.Any(), gomock.Any(), "python", "-m", "pip", "install", "pip", "--exists-action=w", "--no-index", "--ignore-installed", "--find-links=/tmp/pip")
 				mockStager.EXPECT().LinkDirectoryInDepDir(filepath.Join(filepath.Join(depDir, "python"), "bin"), "bin")