main: add error tests for the compiler

The compiler can in fact return errors, but these weren't tested yet.

errors_test.go

@@ -17,6 +17,7 @@ import (
 func TestErrors(t *testing.T) {
 	for _, name := range []string{
 		"cgo",
+		"compiler",
 		"interp",
 		"loader-importcycle",
 		"loader-invaliddep",

testdata/errors/compiler.go

@@ -0,0 +1,13 @@
+package main
+
+//go:wasmimport foo bar
+func foo() {
+}
+
+//go:align 7
+var global int
+
+// TODO: include package name in the error message
+
+// ERROR: compiler.go:4:6: can only use //go:wasmimport on declarations
+// ERROR: compiler.go:8:5: global variable alignment must be a positive power of two