KLC check F6.3: Heatsink pads should have solid zone connections

This adds a check for the KLC rule in F6.3 that says heatsink pads should have "solid" zone connections.

Note that this does not check that an EP has the heatsink property in the first place (that would be a separate check).

Also update the makefile test target to do exactly what the gitlab CI does, then make the gitlab CI use the makefil target. This means local testing can exactly match gitlab CI.

Example of a failing test (This is a modified footprint to add heatsink property, but not the zone connection - as most heatsink pads are set correctly because only generated footprints usually set heatsink properties at all, there aren't many violations right now):

Checking footprint 'Cypress_QFN-56-1EP_8x8mm_P0.5mm_EP6.22x6.22mm_ThermalVias':
Violating F6.3 - https://klc.kicad.org/footprint/f6/f6.3/
Pad requirements for SMD footprints
Rectangular SMD pad
- Pads 57 are rectangular. If possible, change to rounded-rectangle.
Pad(s) potentially missing layers
- Pad '57' missing layer 'Mask'
- Pad '57' missing layer 'Mask'
Heatsink pads are missing 'solid' zone connection: 57