add test to test all the flag types

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Prerequisite

• Please consider implementing the feature as a hook script or plugin as a first step.
  • pyenv has some powerful support for plugins and hook scripts. Please refer to Authoring plugins for details and try to implement it as a plugin if possible.
• Please consider contributing the patch upstream to rbenv, since we have borrowed most of the code from that project.
  • We occasionally import the changes from rbenv. In general, you can expect changes made in rbenv will be imported to pyenv too, eventually.
  • Generally speaking, we prefer not to make changes in the core in order to keep compatibility with rbenv.
• My PR addresses the following pyenv issue (if any)
  • Closes Add PACKAGE_LDFLAGS to python-build #2926

Description

• Here are some details about my PR
  Allow to specify subj to complement existing PACKAGE_CFLAGS

Tests

• My PR adds the following unit tests (if any)
  Test to test all the flags.
  Removed test that tested only some of the flags