testscript: allow quoting of regexp metacharacters in env vars (#31)

testscript/doc.go

@@ -87,6 +87,12 @@ quote indicates a literal single quote, as in:
 A line beginning with # is a comment and conventionally explains what is
 being done or tested at the start of a new phase in the script.
 
+A special form of environment variable syntax can be used to quote
+regexp metacharacters inside environment variables. The "@R" suffix
+is special, and indicates that the variable should be quoted.
+
+	${VAR@R}
+
 The command prefix ! indicates that the command on the rest of the line
 (typically go or a matching predicate) must fail, not succeed. Only certain
 commands support this prefix. They are indicated below by [!] in the synopsis.

testscript/scripts/regexpquote.txt

@@ -0,0 +1,5 @@
+env XXX='hello)'
+grep ^${XXX@R}$ file.txt
+
+-- file.txt --
+hello)

testscript/testscript.go

@@ -477,7 +477,12 @@ func (ts *TestScript) Exec(command string, args ...string) error {
 
 // expand applies environment variable expansion to the string s.
 func (ts *TestScript) expand(s string) string {
-	return os.Expand(s, func(key string) string { return ts.envMap[key] })
+	return os.Expand(s, func(key string) string {
+		if key1 := strings.TrimSuffix(key, "@R"); len(key1) != len(key) {
+			return regexp.QuoteMeta(ts.envMap[key1])
+		}
+		return ts.envMap[key]
+	})
 }
 
 // fatalf aborts the test with the given failure message.