Test failures in MERSI and VIIRS readers after fixing bugs in test routines

[gerritholl]

Describe the bug

PR #1270 replaces all incorrect usages of self.assertTrue by self.assertEquals, where the latter was clearly the intention. This has lead to test failures in the readers for MERSI2 L1B and VIIRS SDR:

FAILED reader_tests/test_mersi2_l1b.py::TestMERSI2L1BReader::test_fy3d_1km_resolutions - AssertionError: 4 != 2
FAILED reader_tests/test_mersi2_l1b.py::TestMERSI2L1BReader::test_fy3d_250_resolutions - AssertionError: 4 != 2
FAILED reader_tests/test_viirs_sdr.py::TestVIIRSSDRReader::test_init_end_time_beyond - AssertionError: 1 != 0
FAILED reader_tests/test_viirs_sdr.py::TestVIIRSSDRReader::test_init_start_time_beyond - AssertionError: 1 != 0

Those tests should have been failing all along, they were erroneously passing due to the erroneous usage of self.assertTrue, so this bug is not new in #1270, it was just unmasked there.

To Reproduce

• Replace all incorrect usages of self.assertTrue by self.assertEquals, such as done in Replace incorrect assertTrue usage in unittests #1270
• Run test suite

Expected behavior

All tests should pass.

Actual results

Some tests are failing, see above.

Environment Info:

• Satpy Version: 0.22.1.dev48+gdf478972.d20200723
• Readers and writers dependencies (when relevant): All applicable readers reported ok.