Move unit tests to the location Meson expects them to be

Everything that is a separate subproject should live in the subprojects
directory.

Progress on #2503

This reverts commit 451f8a8c19.

src/libutil-tests/spawn.cc

@@ -0,0 +1,36 @@
+#include <gtest/gtest.h>
+
+#include "processes.hh"
+
+namespace nix {
+
+#ifdef _WIN32
+TEST(SpawnTest, spawnEcho)
+{
+    auto output = runProgram(RunOptions{.program = "cmd.exe", .args = {"/C", "echo", "hello world"}});
+    ASSERT_EQ(output.first, 0);
+    ASSERT_EQ(output.second, "\"hello world\"\r\n");
+}
+
+std::string windowsEscape(const std::string & str, bool cmd);
+
+TEST(SpawnTest, windowsEscape)
+{
+    auto empty = windowsEscape("", false);
+    ASSERT_EQ(empty, R"("")");
+    // There's no quotes in this argument so the input should equal the output
+    auto backslashStr = R"(\\\\)";
+    auto backslashes = windowsEscape(backslashStr, false);
+    ASSERT_EQ(backslashes, backslashStr);
+
+    auto nestedQuotes = windowsEscape(R"(he said: "hello there")", false);
+    ASSERT_EQ(nestedQuotes, R"("he said: \"hello there\"")");
+
+    auto middleQuote = windowsEscape(R"( \\\" )", false);
+    ASSERT_EQ(middleQuote, R"(" \\\\\\\" ")");
+
+    auto space = windowsEscape("hello world", false);
+    ASSERT_EQ(space, R"("hello world")");
+}
+#endif
+}