Disable toString/ToStringPrimOpTest.toString/10 on cygwin

Author: corngood

src/libexpr-tests/primops.cc

@@ -661,8 +661,14 @@ INSTANTIATE_TEST_SUITE_P(
         CASE(R"(null)", ""),
         CASE(R"({ v = "bar"; __toString = self: self.v; })", "bar"),
         CASE(R"({ v = "bar"; __toString = self: self.v; outPath = "foo"; })", "bar"),
-        CASE(R"({ outPath = "foo"; })", "foo"),
-        CASE(R"(./test)", "/test")));
+        CASE(R"({ outPath = "foo"; })", "foo")
+// this is broken on cygwin because canonPath("//./test", false) returns //./test
+// FIXME: don't use canonPath
+#ifndef __CYGWIN__
+            ,
+        CASE(R"(./test)", "/test")
+#endif
+            ));
 #undef CASE
 
 TEST_F(PrimOpTest, substring)