Add expression testing values for log_diff_exp

[andrjohns]

Summary

The log_diff_exp function will fail in expression tests with the default values as nan values are returned, this PR specifies values to test so that a valid result is returned

Tests

N/A

Side Effects

N/A

Release notes

N/A

Checklist

• Math issue #(issue number)

• Copyright holder: Andrew Johnson

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	4.11	4.33	0.95	-5.23% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.96	-4.04% slower
eight_schools/eight_schools.stan	0.09	0.09	1.0	-0.43% slower
gp_regr/gp_regr.stan	0.2	0.2	1.04	3.5% faster
irt_2pl/irt_2pl.stan	5.23	5.25	1.0	-0.36% slower
performance.compilation	88.49	87.46	1.01	1.16% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.26	8.21	1.01	0.61% faster
pkpd/one_comp_mm_elim_abs.stan	27.69	27.24	1.02	1.65% faster
sir/sir.stan	139.84	130.3	1.07	6.82% faster
gp_regr/gen_gp_data.stan	0.05	0.05	1.04	3.87% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.42	3.33	1.03	2.51% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.4	0.38	1.03	3.05% faster
arK/arK.stan	2.54	2.58	0.99	-1.38% slower
arma/arma.stan	0.73	0.73	0.99	-0.85% slower
garch/garch.stan	0.72	0.72	1.0	0.11% faster
Mean result: 1.0082822895

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Jenkins Console Log
Blue Ocean
Commit hash: 94d87c7

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Machine information

ProductName: Mac OS X ProductVersion: 10.11.6 BuildVersion: 15G22010

CPU:
Intel(R) Xeon(R) CPU E5-1680 v2 @ 3.00GHz

G++:
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

Clang:
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix