Add ruby_install_name helper

RubyGems 2.2.1 causes CI failures on linux due to nil values in
RbConfig::CONFIG.  This is due to RubyGems setting RbConfig values to
nil when they should not be set at all.
Add ENABLE_SHARED helper

See @7128302 for a description
Add configure_args helper

See @7128302 for a description
Only set bindir when previously set

See @7128302 for a description
Only reset RUBY_SO_NAME when it was set before

See @7128302 for a description
Pull enable_shared helper up to TestCase

This functionality can be reused.
Use enable_shared helper for test_require_paths

See @7128302 for a description