Moves test-only symbols out of wazero package (#597)

This moves the compiler support flag out of the public package as it was
only put there for tests. This also files modgen under the testing
subdir so that it isn't mistaken for main code.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

config_supported.go

@@ -2,9 +2,6 @@
 
 package wazero
 
-// CompilerSupported returns whether the compiler is supported in this environment.
-const CompilerSupported = true
-
 // NewRuntimeConfig returns NewRuntimeConfigCompiler
 func NewRuntimeConfig() RuntimeConfig {
 	return NewRuntimeConfigCompiler()