Fix tests with serde >= 1.0.196

[jamessan]

In serde 1.0.196, the "invalid type" error message for floats started
adding a trailing ".0" if no decimal point was already part of the
formatted float, causing tests to fail like:

---- test_chrono_timestamp_seconds_with_frac stdout ----

error: expect test failed
   --> serde_with/tests/chrono_0_4.rs:551:9

Expect:
----
invalid type: floating point `0`, expected a string at line 1 column 3
----

Actual:
----
invalid type: floating point `0.0`, expected a string at line 1 column 3
----

Change the tests to use a test value of 0.1, so the test will work both
pre- and post-serde 1.0.196.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.55%. Comparing base (04de4ce) to head (1e4481c).
Report is 17 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #740   +/-   ##
=======================================
  Coverage   66.55%   66.55%           
=======================================
  Files          38       38           
  Lines        2473     2473           
=======================================
  Hits         1646     1646           
  Misses        827      827           

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[jonasbb]

Thanks for the contribution.