Expand unit testing for more cases

[awvwgk]

@zoziha @St-Maxwell Here is suggestion for expanding the test suite for the format_string PR.

 format_string(complex) : 
 - Default formatter for complex number
   Result: '(1.00000000,1.00000000)'
 - Formatter for complex number
   Result: '(  1.00,  1.00)'
 - Formatter for negative complex number
   Result: '( -1.00, -1.00)'
 - Formatter with sign control descriptor for complex number
   Result: '( +1.00, +1.00)'
 - Multiple formatters for complex numbers
   Result: '(  1.00,  1.00)(  2.000,  2.000)'
 format_string(integer) : 
 - Default formatter for integer number
   Result: '100'
 - Formatter for integer number
   Result: '   100'
 - Formatter with zero padding for integer number
   Result: '000100'
 - Multiple formatters for integers
   Result: '   100   1000'
 - Binary formatter for integer number
   Result: '  100010'
 - Octal formatter with zero padding for integer number
   Result: '042'
 - Hexadecimal formatter for integer number
   Result: ' 22'
 format_string(real) : 
 - Default formatter for real number
   Result: '100.000000'
 - Formatter for real number
   Result: '100.00'
 - Exponential formatter with rounding for real number
   Result: '.29E+03'
 - Exponential formatter for real number
   Result: '1.28E+02'
 - Too narrow formatter for signed real number
   Result: '******'
 - Too narrow formatter for real number
   Result: '******'
 - Real formatter for integer number
   Result: '*'
 format_string(logical) : 
 - Default formatter for logcal value
   Result: 'T'
 - Formatter for logical value
   Result: ' T'
 - Multiple formatters for logical values
   Result: ' F    T'
 - Integer formatter for logical value
   Result: '*'

[St-Maxwell]

👍Good idea. The test cases for binary, octal and hexadecimal format are inspiring.