testscript: ensure that temp dir isn't sym-linked (#79)

In cases where ioutil.TempDir() returns a symbolic link (default
behaviour in macOS) the workdir substitution could mess up
output matching for programs that uses the real dir.

This PR introduces sym-link evaluation of the test temp dir to
ensure consistency.

Author: leitzler

testscript/testdata/evalsymlink.txt

@@ -0,0 +1,8 @@
+# If ioutil.TempDir returns a sym linked dir (default behaviour in macOS for example) the
+# matcher will have problems with external programs that uses the real path.
+# This script tests that $WORK is matched in a consistent way (also see #79).
+[windows] skip
+exec pwd
+stdout ^$WORK$
+exec pwd -P
+stdout ^$WORK$

testscript/testscript.go

@@ -150,6 +150,14 @@ func RunT(t T, p Params) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	// The temp dir returned by ioutil.TempDir might be a sym linked dir (default
+	// behaviour in macOS). That could mess up matching that includes $WORK if,
+	// for example, an external program outputs resolved paths. Evaluating the
+	// dir here will ensure consistency.
+	testTempDir, err = filepath.EvalSymlinks(testTempDir)
+	if err != nil {
+		t.Fatal(err)
+	}
 	refCount := int32(len(files))
 	for _, file := range files {
 		file := file