Fix unit test fff_print_tests

[jalapenopuzzle]

fff_print_tests fail when run in Debug mode:
At commit 5c7888b

user@prusa:/home/user/PrusaSlicer/build-debug$ make test
Running tests...
Test project /home/user/PrusaSlicer/build-debug
    Start 1: arrange_tests
1/6 Test #1: arrange_tests ....................   Passed   34.79 sec
    Start 2: thumbnails_tests
2/6 Test #2: thumbnails_tests .................   Passed    0.39 sec
    Start 3: libslic3r_tests
3/6 Test #3: libslic3r_tests ..................Subprocess aborted***Exception: 100.95 sec
    Start 4: fff_print_tests
4/6 Test #4: fff_print_tests ..................Subprocess aborted***Exception: 376.86 sec
    Start 5: sla_print_tests
5/6 Test #5: sla_print_tests ..................   Passed  3161.06 sec
    Start 6: slic3rutils_tests
6/6 Test #6: slic3rutils_tests ................   Passed   16.77 sec

67% tests passed, 2 tests failed out of 6

Total Test time (real) = 3690.87 sec

The following tests FAILED:
	  3 - libslic3r_tests (Subprocess aborted)
	  4 - fff_print_tests (Subprocess aborted)
Errors while running CTest
Output from these tests are in: /home/user/PrusaSlicer/build-debug/Testing/Temporary/LastTest.log
Use "--rerun-failed --output-on-failure" to re-run the failed cases verbosely.
make: *** [Makefile:71: test] Error 8
user@prusa:/home/user/PrusaSlicer/build-debug$ 

This pull request addresses the failures in the fff_print_tests.

user@prusa:/home/user/PrusaSlicer/build-debug$ make test
Running tests...
Test project /home/user/PrusaSlicer/build-debug
    Start 1: arrange_tests
1/6 Test #1: arrange_tests ....................   Passed   31.95 sec
    Start 2: thumbnails_tests
2/6 Test #2: thumbnails_tests .................   Passed    0.03 sec
    Start 3: libslic3r_tests
3/6 Test #3: libslic3r_tests ..................Subprocess aborted***Exception:  94.54 sec
    Start 4: fff_print_tests
4/6 Test #4: fff_print_tests ..................   Passed  1178.31 sec
    Start 5: sla_print_tests
5/6 Test #5: sla_print_tests ..................   Passed  3006.60 sec
    Start 6: slic3rutils_tests
6/6 Test #6: slic3rutils_tests ................   Passed   16.55 sec

83% tests passed, 1 tests failed out of 6

Total Test time (real) = 4328.06 sec

The following tests FAILED:
	  3 - libslic3r_tests (Subprocess aborted)
Errors while running CTest
Output from these tests are in: /home/user/PrusaSlicer/build-debug/Testing/Temporary/LastTest.log
Use "--rerun-failed --output-on-failure" to re-run the failed cases verbosely.
make: *** [Makefile:71: test] Error 8
user@prusa:/home/user/PrusaSlicer/build-debug$ 

Tested on Ubuntu 22.04 virtual machine.

The following changes were made:

• Fix fff_print_tests assertion failure axis.size() <= 1
  PrusaSlicer/src/libslic3r/GCodeReader.cpp:24: char Slic3r::get_extrusion_axis_char(const Slic3r::GCodeConfig&): Assertion `axis.size() <= 1' failed.
  The set_deserialise_strict() method converts 'A' to the string "65" (representing the ASCII numerical value) instead of the string "A". Perhaps this should be fixed more robustly.
• Fix fff_print_tests test name Temeperatures -> Temperatures
• Fix fff_print_tests All travel moves happen within skirt
  Remove the last travel_moves point which returns to the origin (0,0) which is outside the convex hull. This point was causing CHECK(convex_hull.contains(travel_move)) to fail.
• Fix fff_print_tests "Used Filament" test Assertion std::abs(length) < 1000.0' failed The test configured a retract_length of 10000000 which is larger than the asserted maximum retraction length of 1000. PrusaSlicer/src/libslic3r/GCode/GCodeWriter.cpp:473: std::string Slic3r::GCodeWriter::_retract(double, double, std::string_view): Assertion std::abs(length) < 1000.0' failed.
  Fixed by doing the print with two different (legal) retraction lengths and checking that the total_used_filament agrees in both cases.
• Fix fff_print_tests "Slicing with retraction and lifting" remove illegal negative restart_extra test case
  Negative restart_extra is asserted against at PrusaSlicer/src/libslic3r/Extrucer.cpp:58
• How to build - add instructions on how to run the unit tests