Check result from test resource access

[JSAbrahams]

Relevant issues

One issue in #61 is that when looking at the Travis build it was not easy to deduce what went wrong since we don't output what happens when accessing test resources goes awry. Furthermore, this works towards #63 .

Summary

• Remove some useless calls to assert_eq! by simply using unwrap_err(), which panics if we do not have an error. This is relevant for the invalid parser tests, which verify that incorrect syntax results in a ParseErr.
• Add helpful panic messages for when accessing test resources goes awry.
• Remove ugly assert_ok! macro

Added Tests

No tests added, but some existing tests have been simplified.

Additional Context

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[codecov[bot]]

Codecov Report

Merging #67 into master will decrease coverage by 0.36%. The diff coverage is 91.3%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
- Coverage   80.09%   79.72%   -0.37%     
==========================================
  Files          57       57              
  Lines        3893     3847      -46     
==========================================
- Hits         3118     3067      -51     
- Misses        775      780       +5

Impacted Files	Coverage Δ
src/parser/mod.rs	100% <ø> (ø)	:arrow_up:
tests/parser/valid/control_flow.rs	87.07% <100%> (ø)	:arrow_up:
tests/parser/invalid/operation.rs	100% <100%> (ø)	:arrow_up:
tests/parser/valid/collection.rs	86.13% <100%> (ø)	:arrow_up:
tests/parser/mod.rs	100% <100%> (ø)	:arrow_up:
tests/parser/invalid/function.rs	100% <100%> (ø)	:arrow_up:
tests/parser/valid/compound.rs	100% <100%> (ø)	:arrow_up:
tests/parser/invalid/expression_and_statement.rs	100% <100%> (ø)	:arrow_up:
tests/main.rs	93.75% <100%> (+18.75%)	:arrow_up:
tests/parser/invalid/compound.rs	100% <100%> (ø)	:arrow_up:
... and 6 more

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