tests/ffi_float: Split tgammaf() testcase to a separate test.

Some libc's may implement tgammaf as a header macro using tgamma(), so
don't assume it'll be in the library.

tests/unix/ffi_float.py

@@ -28,7 +28,6 @@ print('%.6f' % strtod('1.23', None))
 # test passing double and float args
 libm = ffi_open(('libm.so', 'libm.so.6', 'libc.so.0', 'libc.so.6', 'libc.dylib'))
 tgamma = libm.func('d', 'tgamma', 'd')
-tgammaf = libm.func('f', 'tgammaf', 'f')
-for fun in (tgamma, tgammaf):
+for fun in (tgamma,):
     for val in (0.5, 1, 1.0, 1.5, 4, 4.0):
         print('%.6f' % fun(val))

tests/unix/ffi_float.py.exp

@@ -6,9 +6,3 @@
 0.886227
 6.000000
 6.000000
-1.772454
-1.000000
-1.000000
-0.886227
-6.000000
-6.000000

tests/unix/ffi_float2.py

+# test ffi float support
+import sys
+try:
+    import ffi
+except ImportError:
+    print("SKIP")
+    sys.exit()
+
+
+def ffi_open(names):
+    err = None
+    for n in names:
+        try:
+            mod = ffi.open(n)
+            return mod
+        except OSError as e:
+            err = e
+    raise err
+
+libm = ffi_open(('libm.so', 'libm.so.6', 'libc.so.0', 'libc.so.6', 'libc.dylib'))
+
+# Some libc's implement tgammaf as header macro with tgamma(), so don't assume
+# it'll be in library.
+try:
+    tgammaf = libm.func('f', 'tgammaf', 'f')
+except OSError:
+    print("SKIP")
+    sys.exit()
+
+for fun in (tgammaf,):
+    for val in (0.5, 1, 1.0, 1.5, 4, 4.0):
+        print('%.6f' % fun(val))

tests/unix/ffi_float2.py.exp

+1.772454
+1.000000
+1.000000
+0.886227
+6.000000
+6.000000