ptest: Don't output colors unconditionally

[nmeum]

Started using ptest for an mpc-parser because it was used in mpc itself.
While running my ptest parser test suite on a CI I noticed that ptest
outputs colors unconditionally. This change only outputs colors if the
TERM variable is not set to dumb and standard output is a TTY.

I obviously didn't test this change on WIN32.

---

Also make outputs a bunch of errors on my system currently:

example.c: In function 'suite_basic':
example.c:5:11: error: trampoline generated for nested function 'test_maths' [-Werror=trampolines]
    5 |   PT_TEST(test_maths) {
      |           ^~~~~~~~~~
ptest.h:10:64: note: in definition of macro 'PT_TEST'
   10 | #define PT_TEST(name) auto void name(void); PT_REG(name); void name(void)
      |                                                                ^~~~
example.c:14:11: error: trampoline generated for nested function 'test_strings' [-Werror=trampolines]
   14 |   PT_TEST(test_strings) {
      |           ^~~~~~~~~~~~
ptest.h:10:64: note: in definition of macro 'PT_TEST'
   10 | #define PT_TEST(name) auto void name(void); PT_REG(name); void name(void)
      |                                                                ^~~~
example.c: In function 'suite_other':
example.c:22:11: error: trampoline generated for nested function 'test_stuff' [-Werror=trampolines]
   22 |   PT_TEST(test_stuff) {
      |           ^~~~~~~~~~
ptest.h:10:64: note: in definition of macro 'PT_TEST'
   10 | #define PT_TEST(name) auto void name(void); PT_REG(name); void name(void)
      |                                                                ^~~~
example.c:28:11: error: trampoline generated for nested function 'test_failure' [-Werror=trampolines]
   28 |   PT_TEST(test_failure) {
      |           ^~~~~~~~~~~~
ptest.h:10:64: note: in definition of macro 'PT_TEST'
   10 | #define PT_TEST(name) auto void name(void); PT_REG(name); void name(void)
      |                                                                ^~~~
cc1: all warnings being treated as errors
make: *** [Makefile:8: example] Error 1