tests/thirdparty: fix and run as github workflow (#198)

As part of fixing failing third-party tests, this PR significantly
rearchitects their specification and execution.

Third-party tests are now specified in a much more flexible format allowing
more customization on a per-package level. In addition, third-party tests are
now used to auto-generate a Github Actions workflow to perform the tests in
parallel. This not only gives faster feedback on PRs, but will also allow us
to more quickly narrow down on which packages are failing. An additional
workflow also confirms that local execution of third-party tests is consistent
with the Github Actions version. This workflow only runs when tests/thirdparty
itself is changed.

internal/prnt/printer.go

@@ -13,8 +13,11 @@ import (
 // any errors to be stored so they can be checked at the end, rather than having
 // error checks obscuring the code generation.
 type Generator struct {
-	buf bytes.Buffer
-	err error
+	buf     bytes.Buffer
+	level   int    // current indentation level
+	indent  string // indentation string
+	pending bool   // if there's a pending indentation
+	err     error  // saved error from printing
 }
 
 // Raw provides direct access to the underlying output stream.
@@ -22,11 +25,38 @@ func (g *Generator) Raw() io.Writer {
 	return &g.buf
 }
 
+// SetIndentString sets the string used for one level of indentation. Use
+// Indent() and Dedent() to control indent level.
+func (g *Generator) SetIndentString(indent string) {
+	g.indent = indent
+}
+
+// Indent increments the indent level.
+func (g *Generator) Indent() {
+	g.level++
+}
+
+// Dedent decrements the indent level.
+func (g *Generator) Dedent() {
+	g.level--
+}
+
+// Linef prints formatted output terminated with a new line.
+func (g *Generator) Linef(format string, args ...interface{}) {
+	g.Printf(format, args...)
+	g.NL()
+}
+
 // Printf prints to the internal buffer.
 func (g *Generator) Printf(format string, args ...interface{}) {
 	if g.err != nil {
 		return
 	}
+	if g.pending {
+		indent := strings.Repeat(g.indent, g.level)
+		format = indent + format
+		g.pending = false
+	}
 	_, err := fmt.Fprintf(&g.buf, format, args...)
 	g.AddError(err)
 }
@@ -34,6 +64,7 @@ func (g *Generator) Printf(format string, args ...interface{}) {
 // NL prints a new line.
 func (g *Generator) NL() {
 	g.Printf("\n")
+	g.pending = true
 }
 
 // Comment writes comment lines prefixed with "// ".