feat: improve test coverage (~1%)

[nigrosimone]

I attempted an optimization and all tests are passing, but only because some cases are currently missing. This PR improves test coverage to prevent similar issues in the future.

This PR adds only tests (no benchmark needed)

MASTER

----------------------------|---------|----------|---------|---------|----------------------------
File                        | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------------------------|---------|----------|---------|---------|----------------------------
All files                   |   81.48 |    69.12 |   31.12 |   81.48 |                            
 fast-json-stringify        |    96.7 |    95.16 |     100 |    96.7 |                            
  index.js                  |    96.7 |    95.16 |     100 |    96.7 | ...605-606,613-640,968-969 
 fast-json-stringify/.cache |   42.85 |    50.77 |   25.85 |   42.85 |                            
  webpacktest.js            |   42.85 |    50.77 |   25.85 |   42.85 | 2-3,5-6                    
 fast-json-stringify/lib    |   68.49 |    73.59 |    92.3 |   68.49 |                            
  location.js               |     100 |      100 |     100 |     100 |                            
  merge-schemas.js          |     100 |      100 |     100 |     100 |                            
  schema-validator.js       |   61.02 |    61.88 |     100 |   61.02 | ...058,1068-1070,1078-1084 
  serializer.js             |   97.87 |    98.59 |    90.9 |   97.87 | 96,139-140                 
  standalone.js             |     100 |      100 |     100 |     100 |                            
  validator.js              |   91.66 |    90.47 |   85.71 |   91.66 | 36-37,88-93                
----------------------------|---------|----------|---------|---------|----------------------------

PR

----------------------------|---------|----------|---------|---------|----------------------------
File                        | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------------------------|---------|----------|---------|---------|----------------------------
All files                   |   82.74 |    69.81 |   31.41 |   82.74 |                            
 fast-json-stringify        |   96.86 |    95.49 |     100 |   96.86 |                            
  index.js                  |   96.86 |    95.49 |     100 |   96.86 | ...597-602,605-606,613-640 
 fast-json-stringify/.cache |   42.85 |    50.77 |   25.85 |   42.85 |                            
  webpacktest.js            |   42.85 |    50.77 |   25.85 |   42.85 | 2-3,5-6                    
 fast-json-stringify/lib    |   70.72 |    75.47 |     100 |   70.72 |                            
  location.js               |     100 |      100 |     100 |     100 |                            
  merge-schemas.js          |     100 |      100 |     100 |     100 |                            
  schema-validator.js       |   63.13 |    65.11 |     100 |   63.13 | ...000,1008-1010,1020-1022 
  serializer.js             |   99.29 |    98.61 |     100 |   99.29 | 96                         
  standalone.js             |     100 |      100 |     100 |     100 |                            
  validator.js              |   97.91 |     90.9 |     100 |   97.91 | 36-37                      
----------------------------|---------|----------|---------|---------|----------------------------

[gurgunday]

lgtm