doc(test/README): add usage information

This commit adds the following usage information to README.md and implements
the described functionality in test/main.f90:

Executing `fpm test -- --help` prints the following message:

Usage: fpm test -- [--help] | [--contains <substring>]

where square brackets ([]) denote optional arguments, a pipe (|) separates alternative arguments,
angular brackets (<>) denote a user-provided value, and passing a substring limits execution to
the tests with test subjects or test descriptions containing the user-specified substring.

Author: rouson

README.md

@@ -82,6 +82,16 @@ git clone [email protected]:sourceryinstitute/sourcery
 
 Building and Testing
 --------------------
+### Test-Suite Usage
+Executing `fpm test -- --help` prints the following message:
+```
+Usage: fpm test -- [--help] | [--contains <substring>]
+
+where square brackets ([]) denote optional arguments, a pipe (|) separates alternative arguments,
+angular brackets (<>) denote a user-provided value, and passing a substring limits execution to
+the tests with test subjects or test descriptions containing the user-specified substring.
+```
+
 ### Single-image (serial) testing with GNU Fortran (`gfortran`) 
 With recent versions of [GNU Fortran] (gfortran) and [OpenCoarrays] installed, 
 execute the following command in a `zsh` or `bash`-like shell:

test/main.f90

@@ -7,6 +7,7 @@ program main
   use test_result_test_m, only : test_result_test_t  
   use command_line_test_m, only : command_line_test_t
   use string_test_m, only : string_test_t
+  use sourcery_m, only : command_line_t
   implicit none
 
   type(collectives_test_t) collectives_test
@@ -20,6 +21,18 @@ program main
 
   integer :: passes=0, tests=0
 
+  block 
+    type(command_line_t) command_line
+    character(len=*), parameter :: usage = &
+      new_line('') // new_line('') // &
+      'Usage: fpm test -- [--help] | [--contains <substring>]' // &
+      new_line('') // new_line('') // &
+      'where square brackets ([]) denote optional arguments, a pipe (|) separates alternative arguments,' // new_line('') // &
+      'angular brackets (<>) denote a user-provided value, and passing a substring limits execution to' // new_line('') // &
+      'the tests with test subjects or test descriptions containing the user-specified substring.' // new_line('') 
+    if (command_line%argument_present([character(len=len("--help"))::"--help","-h"])) stop usage
+  end block
+
   call bin_test%report(passes, tests)
   call data_partition_test%report(passes, tests)
   call collectives_test%report(passes, tests)