Exporting MakeBazelTarget() in testing.go

Since we are putting conversion tests in the same
package as the module being converted from now on

Test: m bp2build
Change-Id: Ied53589d497fef70bfa11d8f6067459c9a50cd72

bp2build/linker_config_conversion_test.go

@@ -37,7 +37,7 @@ linker_config {
 	src: "a.json",
 }
 `,
-			ExpectedBazelTargets: []string{makeBazelTarget("linker_config", "foo", AttrNameToString{
+			ExpectedBazelTargets: []string{MakeBazelTarget("linker_config", "foo", AttrNameToString{
 				"src": `"a.json"`,
 			})},
 		})