Add convenience functions to Python test utilities for rendering with map settings

- Update labeling unit tests to use new functions
- All labeling test classes render separately from QgsRenderChecker
- Update labeling server test class to sync map settings to temp project
Add ability to define QImage format for render jobs via map settings

- Defaults to QImage::Format_ARGB32_Premultiplied, as before
- Update labeling unit tests to use defined format
Note about PdfFormat and NativeFormat for QPrinter PDF output and embedding fonts

- Unlike the Qt issues for NativeFormat on Mac, choosing to print, then Save as PDF, embeds fonts
Initial fix for #3975, label engine vectorizing texts in SVG and PDF output

- Add labeling engine option to render text-as-text
- Default is still text-as-outlines (vectorized), due to differences between text (as text) and buffer (as outline) methods
- Good output with printing to PDF (searchable, selectable text and embedded fonts)
- OK output with SVG, but differences between text (as text) and buffer (as outline) methods

Does not yet include unit tests or auto-setting of text-as-text for SVG output
Fix local test server response parsing and add debug output for url
Merge pull request #1286 from dakcarto/issue-3975_labeling-svg-as-text

Fix for issue 3975, labeling svg as text