test: fix RunTest on systems with single file core utilities (#95)

On systems such as Alpine Linux (with busybox) and Adelie Linux (with
coreutils), the shell core utilities such as pwd, true, false, env, and
yes are all symlinks to a single binary. This single binary relies on
the name of the symlink to determine which command ("applet" in busybox
parlance) to execute. Therefore creating symlinks to these symlinks will
not work since the single binary will only see the top-level symlink and
thus think it is an invalid command.

Instead, generate executable shell scripts that exec into the desired
command. This also allows $PATH based resolution to occur instead of
hard-coding the command paths.

See also issue #94.

test/test-run.cpp

@@ -68,13 +68,16 @@ protected:
     TempDir tmp;
     class Run run;
 
-    void setRunLink(const char* path) {
-        tmp.fs->symlink(kj::Path{"cfg","jobs",run.name+".run"}, path, kj::WriteMode::CREATE|kj::WriteMode::CREATE_PARENT|kj::WriteMode::EXECUTABLE);
+    void setRunLink(const char * path) {
+        KJ_IF_MAYBE(f, tmp.fs->tryOpenFile(kj::Path{"cfg", "jobs", run.name + ".run"},
+                kj::WriteMode::CREATE | kj::WriteMode::CREATE_PARENT | kj::WriteMode::EXECUTABLE)) {
+            (f->get())->writeAll(std::string("#!/bin/sh\nexec ") + path + "\n");
+        }
     }
 };
 
 TEST_F(RunTest, WorkingDirectory) {
-    setRunLink("/bin/pwd");
+    setRunLink("pwd");
     run.configure(1, node, *tmp.fs);
     runAll();
     std::string cwd{tmp.path.append(kj::Path{"run","foo","1"}).toString(true).cStr()};
@@ -82,21 +85,21 @@ TEST_F(RunTest, WorkingDirectory) {
 }
 
 TEST_F(RunTest, SuccessStatus) {
-    setRunLink("/bin/true");
+    setRunLink("true");
     run.configure(1, node, *tmp.fs);
     runAll();
     EXPECT_EQ(RunState::SUCCESS, run.result);
 }
 
 TEST_F(RunTest, FailedStatus) {
-    setRunLink("/bin/false");
+    setRunLink("false");
     run.configure(1, node, *tmp.fs);
     runAll();
     EXPECT_EQ(RunState::FAILED, run.result);
 }
 
 TEST_F(RunTest, Environment) {
-    setRunLink("/usr/bin/env");
+    setRunLink("env");
     run.configure(1234, node, *tmp.fs);
     runAll();
 
@@ -113,7 +116,7 @@ TEST_F(RunTest, Environment) {
 }
 
 TEST_F(RunTest, ParamsToEnv) {
-    setRunLink("/usr/bin/env");
+    setRunLink("env");
     run.params["foo"] = "bar";
     run.configure(1, node, *tmp.fs);
     runAll();
@@ -122,7 +125,7 @@ TEST_F(RunTest, ParamsToEnv) {
 }
 
 TEST_F(RunTest, Abort) {
-    setRunLink("/usr/bin/yes");
+    setRunLink("yes");
     run.configure(1, node, *tmp.fs);
     run.step();
     usleep(200); // TODO fix